BUG/MEDIUM: mux-quic: fix crash on STOP_SENDING received without SD

BUG/MEDIUM: mux-quic: fix crash on STOP_SENDING received without SD

Abort reason code received on STOP_SENDING is notified to upper layer
since the following commit :
  367ce1ebf3e4cead319a9f01581037c9f0280e77
  MINOR: mux-quic: Set tha SE abort reason when a STOP_SENDING frame is received

However, this causes a crash when a STOP_SENDING is received on a QCS
instance without any stream instantiated. Fix this by checking first if
qcs->sd is not NULL before setting abort code.

This bug can easily be reproduced by emitting a STOP_SENDING as first
frame of a stream.

This should fix github issue #2563.

This does not need to be backported.