BUG/MINOR: quic: handle alloc failure on qc_new_conn() for owned socket

BUG/MINOR: quic: handle alloc failure on qc_new_conn() for owned socket

This patch is the follow up of previous fix :
  BUG/MINOR: quic: properly handle alloc failure in qc_new_conn()

quic_conn owned socket FD is initialized as soon as possible in
qc_new_conn(). This guarantees that we can safely call
quic_conn_release() on allocation failure. This function uses internally
qc_release_fd() to free the socket FD unless it has been initialized to
an invalid FD value.

Without this patch, a segfault will occur if one inner allocation of
qc_new_conn() fails before qc.fd is initialized.

This change is linked to quic-conn owned socket implementation.
This should be backported up to 2.7.