BUG/MEDIUM: mux-quic: properly release conn-stream on detach

On qc_detach(), the qcs must cleared the conn-stream context and set its
cs pointer to NULL. This prevents the qcs to point to a dangling
reference.

Without this, a SEGFAULT may occurs in qc_wake_some_streams() when
accessing an already detached conn-stream instance through a qcs.

Here is the SEGFAULT observed on haproxy.org.
 Program terminated with signal 11, Segmentation fault.
 1234                            else if (qcs->cs->data_cb->wake) {
 (gdb) p qcs.cs.data_cb
 $1 = (const struct data_cb *) 0x0

This can happens since the following patch :
 commit fe035eca3a24ea0f031fdcdad23809bea5de32e4
 MEDIUM: mux-quic: report errors on conn-streams
1 file changed