BUG/MEDIUM: quic-be: crash after backend CID allocation failures

BUG/MEDIUM: quic-be: crash after backend CID allocation failures

This bug impacts only the QUIC backends. It arrived with this commit:
   MINOR: quic-be: QUIC connection allocation adaptation (qc_new_conn())
which was supposed to be fixed by:
   BUG/MEDIUM: quic: crash after quic_conn allocation failures
but this commit was not sufficient.

Such a crashe could be reproduced with -dMfail option. To reach it, the
<conn_id> object allocation must fail (from qc_new_conn()). So, this is
relatively rare, except on systems with limited memory.

No need to backport.