BUG/MEDIUM: h3: do not crash on invalid response status code

BUG/MEDIUM: h3: do not crash on invalid response status code

A crash occurs in h3_resp_headers_send() if an invalid response code is
received from the backend side. Fix this by properly flagging the
connection on error. This will cause a CONNECTION_CLOSE.

This should fix github issue #2422.

Big thanks to ygkim (@yokim-git) for his help and reactivity. Initially,
GDB reported an invalid code source location due to heavy functions
inlining inside h3_snd_buf(). The issue was found after using -Og flag.

This must be backported up to 2.6.