xine: Store pointer to parsed subtitles in the demuxstr struct.

This field already exists in the data structure, it just wasn't
populated before. Previously the only pointer to the parsed subtitles
was the one returned by the function.

mythtv/libs/libmythtv/captions/xine_demux_sputext.cpp

@@ -1232,6 +1232,7 @@ subtitle_t *sub_read_file (demux_sputext_t *demuxstr) {
   }
 #endif
 
+  demuxstr->subtitles = first;
   // No memory leak of 'sub' here.  'Sub' always points to an element in 'first'.
   // NOLINT(clang-analyzer-unix.Malloc)
   return first;