decoder/flac: fixed NULL pointer dereference in CUE code

The function flac_vtrack_tnum() was missing a strrchr()==NULL check.

NEWS

@@ -11,6 +11,7 @@ ver 0.15.6 (2009/??/??)
   - ffmpeg: align the output buffer
   - oggflac: rewind stream after FLAC detection
   - flac: fixed CUE seeking range check
+  - flac: fixed NULL pointer dereference in CUE code
 * output_thread: check again if output is open on PAUSE
 * update: delete ignored symlinks from database
 * database: increased maximum line length to 32 kB

src/decoder/_flac_common.c

@@ -415,6 +415,8 @@ flac_vtrack_tnum(const char* fname)
 	 * another/better way would be to use tag struct
 	 */
 	char* ptr = strrchr(fname, '_');
+	if (ptr == NULL)
+		return 0;
 
 	// copy ascii tracknumber to int
 	char vtrack[4];