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.

(cherry picked from commit 5d2fe1871a1ec4ec68a8ed262f4526e02e8e9fc1)
[wt: dropped H3_EV_TX_FRAME from trace flags]
Signed-off-by: Willy Tarreau <w@1wt.eu>