BUILD: quic: fix wrong potential NULL dereference

BUILD: quic: fix wrong potential NULL dereference

GCC warns about a possible NULL dereference when requeuing a datagram on
the connection socket. This happens due to a MT_LIST_POP to retrieve a
rxbuf instance.

In fact, this can never be NULL there is enough rxbuf allocated for each
thread. Once a thread has finished to work with it, it must always
reappend it.

This issue was introduced with the following patch :
  commit b34d353968db7f646e83871cb6b21a246af84ddc
  BUG/MEDIUM: quic: consume contig space on requeue datagram
As such, it must be backported in every version with the above commit.

This should fix the github CI compilation error.