Hi,

OpenSSL passes out and outlen variable uninitialized to
select_next_proto_cb callback function. If the callback function
returns SSL_TLSEXT_ERR_OK, the caller assumes the callback filled
values in out and outlen and processes as such. Previously, if there
is no overlap in protocol lists, curl code does not fill any values in
these variables and returns SSL_TLSEXT_ERR_OK, which means we are
triggering undefined behavior. valgrind warns this.

This patch fixes this issue by filling HTTP/2 protocol identifier
nghttp2 library supports when there is no overlap. Unlike ALPN, NPN
specification https://technotes.googlecode.com/git/nextprotoneg.html
says that client should select first protocol it supports if there is
no overlap.

Best regards,
Tatsuhiro Tsujikawa

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