BUG/MINOR: h3: missing goto on buf alloc failure

BUG/MINOR: h3: missing goto on buf alloc failure

The following patch introduced proper error management on buffer
allocation failure :
  0abde9dee69fe151f5f181a34e0782ef840abe53
  BUG/MINOR: mux-quic: properly handle buf alloc failure

However, when decoding an empty STREAM frame with just FIN bit set, this
was not done correctly. Indeed, there is a missing goto statement in
case of a NULL buffer check.

This was reported thanks to coverity analysis. This should fix github
issue #2163.

This must be backported up to 2.6.