decoder/flac: fixed compiler warning

Removed the "vtrack" local variable (which triggered a gcc warning
because it was after the newly introduced NULL check), and run
strtol() on the original parameter.

src/decoder/_flac_common.c

@@ -419,9 +419,7 @@ flac_vtrack_tnum(const char* fname)
 		return 0;
 
 	// copy ascii tracknumber to int
-	char vtrack[4];
-	g_strlcpy(vtrack, ++ptr, 4);
-	return (unsigned int)strtol(vtrack, NULL, 10);
+	return (unsigned int)strtol(++ptr, NULL, 10);
 }
 
 #endif /* FLAC_API_VERSION_CURRENT >= 7 */