src/demux.c: send error message (Fixes CID #134924)

The function dvbpsi_DetachDemux(), which is now a wrapper around dvbpsi_chain_demux_delete()
failed to check the return value. Now it returns an error message when the return value is 'false'.

src/demux.c

@@ -97,7 +97,8 @@ void dvbpsi_DetachDemux(dvbpsi_t *p_dvbpsi)
     assert(p_dvbpsi);
     assert(p_dvbpsi->p_decoder);
 
-    dvbpsi_chain_demux_delete(p_dvbpsi);
+    if (!dvbpsi_chain_demux_delete(p_dvbpsi))
+        dvbpsi_error(p_dvbpsi, "demux", "could not free chain_demux decoder resources");
 }
 
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