BUG/MAJOR: quic: fix parsing frame type

BUG/MAJOR: quic: fix parsing frame type

QUIC frame type is encoded as a varint. Initially, haproxy parsed it as
a single byte, which was enough to cover frames defined in RFC9000.

The code has been extended recently to support multi-bytes encoded
value, in anticipation of QUIC frames extension support. However, there
was no check on the varint format. This is interpreted erroneously as a
PADDING frame as this serves as the initial value. Thus the rest of the
packet is incorrectly handled, with various resulting effects, including
infinite loops and/or crashes.

This patch fixes this by checking the return value of quic_dec_int(). If
varint cannot be parsed, the connection is immediately closed.

This issue is assigned to CVE-2026-26080 report.

This must be backported up to 3.2.

Reported-by: Asim Viladi Oglu Manizada <manizada@pm.me>