- stop calling `strcmp()` with NULL to avoid undefined behaviour.
- fix checking results if some of them were NULL.
- do not pass NULL to printf `%s`.

```
In file included from ../../tests/libtest/test.h:43,
                 from curlcheck.h:24,
                 from unit1395.c:24:
unit1395.c: In function ‘test’:
../../lib/curl_printf.h:44:18: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
   44 | # define fprintf curl_mfprintf
unit1395.c:93:7: note: in expansion of macro ‘fprintf’
   93 |       fprintf(stderr, "Test %u: '%s' gave '%s' instead of NULL\n",
      |       ^~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/6896854253/job/18763836775?pr=12346#step:33:1689

```
vssh/libssh.c: In function ‘myssh_statemach_act’:
vssh/libssh.c:1162:29: error: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 2 has type ‘uint64_t’ {aka ‘long unsigned int’} [-Werror=format=]
 1162 |         char *tmp = aprintf("statvfs:\n"
      |                             ^~~~~~~~~~~~
......
 1169 |                             statvfs->f_bsize, statvfs->f_frsize,
      |                             ~~~~~~~~~~~~~~~~
      |                                    |
      |                                    uint64_t {aka long unsigned int}
vssh/libssh.c:1163:42: note: format string is defined here
 1163 |                             "f_bsize: %llu\n" "f_frsize: %llu\n"
      |                                       ~~~^
      |                                          |
      |                                          long long unsigned int
      |                                       %lu
```
Ref: https://github.com/curl/curl/actions/runs/6896854253/job/18763838007?pr=12346#step:29:895

```
In file included from curl_setup.h:656,
                 from conncache.c:26:
conncache.c: In function ‘Curl_conncache_add_conn’:
conncache.c:246:22: error: format ‘%ld’ expects argument of type ‘long int’, but argument 3 has type ‘curl_off_t’ {aka ‘long long int’} [-Werror=format=]
  246 |   DEBUGF(infof(data, "Added connection %ld. "
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~
  247 |                "The cache now contains %zu members",
  248 |                conn->connection_id, connc->num_conn));
      |                ~~~~~~~~~~~~~~~~~~~
      |                    |
      |                    curl_off_t {aka long long int}
curl_setup_once.h:286:19: note: in definition of macro ‘DEBUGF’
  286 | #define DEBUGF(x) x
      |                   ^
conncache.c:246:10: note: in expansion of macro ‘infof’
  246 |   DEBUGF(infof(data, "Added connection %ld. "
      |          ^~~~~
conncache.c:246:42: note: format string is defined here
  246 |   DEBUGF(infof(data, "Added connection %ld. "
      |                                        ~~^
      |                                          |
      |                                          long int
      |                                        %lld
```
Ref: https://github.com/curl/curl/actions/runs/6896854263/job/18763831142?pr=12346#step:6:67

```
http2.c: In function 'nw_out_writer':
http2.c:374:14: error: null pointer dereference [-Werror=null-dereference]
  374 |   nwritten = Curl_conn_cf_send(cf->next, data, (const char *)buf, buflen, err);
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/6896854253/job/18763839238?pr=12346#step:30:214

E.g.:
```
ftpgetinfo.c: In function 'main':
ftpgetinfo.c:78:16: error: ISO C does not support the 'I' printf flag [-Werror=format=]
   78 |         printf("filesize %s: %" CURL_FORMAT_CURL_OFF_T " bytes\n",
      |                ^~~~~~~~~~~~~~~~
ftpgetinfo.c:78:16: error: format '%d' expects argument of type 'int', but argument 3 has type 'curl_off_t' {aka 'long long int'} [-Werror=format=]
   78 |         printf("filesize %s: %" CURL_FORMAT_CURL_OFF_T " bytes\n",
      |                ^~~~~~~~~~~~~~~~
   79 |                filename, filesize);
      |                          ~~~~~~~~
      |                          |
      |                          curl_off_t {aka long long int}
In file included from ../../include/curl/curl.h:54,
                 from ftpgetinfo.c:27:
../../include/curl/system.h:205:42: note: format string is defined here
  205 | #  define CURL_FORMAT_CURL_OFF_T     "I64d"
```
Ref: https://dev.azure.com/daniel0244/curl/_build/results?buildId=18581&view=logs&jobId=ccf9cc6d-2ef1-5cf2-2c09-30f0c14f923b

Caveat: mingw old, I have no info if `inttypes.h` is supported there.

No longer necessary after fixing curl public headers.

The original goal of this option was to test C89-compliance.
Keep that feature, but limited to a specific CI job. We can extended
to other gcc jobs as needed.

Don't add them to these though, as in these revealed actual issues by
building without `-std=gnu89`:

- openssl3-O3
- event-based
- Slackware-openssl-with-gssapi-gcc
- mingw-w64 builds

Applying the same fix as used with libssh.