BUG/MINOR: mux_quic: do not release conn on qcc_recv() for QMux

BUG/MINOR: mux_quic: do not release conn on qcc_recv() for QMux

Recently, an extra check has been added so that a dead connection is
immediately release on at the end of qcc_recv() operation. This is
useful when a GOAWAY frame is received from a server, so that the
backend connection is released if idle.

This step is in fact only necessary for QUIC, as qcc_recv() is called
directly from the lower transport layer. It causes issues with QMux as
in this case qcc_recv() is called via qcc_io_recv(). A crash in this
context will occur as qcc_recv() does not indicate that a release has
been performed.

To fix this, simply disable the extra check at the end of qcc_recv() for
QMux. This is fine as in this case receive operation is always followed
by qcc_io_process() which is able to release the connection in a safe
way.

No need to backport.