BUG/MINOR: mux-quic: check access on qcs stream-endpoint

BUG/MINOR: mux-quic: check access on qcs stream-endpoint

Since the following commit, allocation of stream-endpoint has been
delayed. The objective is to allocate it only for QCS attached to an
upper stream object.

  commit e6064c561684d9b079e3b5725d38dc3b5c1b5cd5
  OPTIM: mux-quic: delay FE sedesc alloc to stream creation

However, some MUX functions are unsafe as qcs->sd is dereferenced
without any check on it which will result in a crash. Fix this by
testing that qcs->sd is allocated before using it.

This does not need to be backported, unless the above patch is.