src/demux.c: Fix CID #110841

Fix dereference before NULL check.

src/demux.c

@@ -220,13 +220,13 @@ void dvbpsi_AttachDemuxSubDecoder(dvbpsi_demux_t *p_demux, dvbpsi_demux_subdec_t
 void dvbpsi_DetachDemuxSubDecoder(dvbpsi_demux_t *p_demux, dvbpsi_demux_subdec_t *p_subdec)
 {
     assert(p_demux);
-    assert(p_demux->p_first_subdec);
-
     assert(p_subdec);
 
     if (!p_demux || !p_subdec)
         abort();
 
+    assert(p_demux->p_first_subdec);
+
     dvbpsi_demux_subdec_t** pp_prev_subdec;
     pp_prev_subdec = &p_demux->p_first_subdec;
     while(*pp_prev_subdec != p_subdec)