qsort cannot be called with NULL

resolves #519

code/qcommon/files.c

@@ -2932,7 +2932,9 @@ void FS_AddGameDirectory( const char *path, const char *dir ) {
 	// Get .pk3 files
 	pakfiles = Sys_ListFiles(curpath, ".pk3", NULL, &numfiles, qfalse);
 
-	qsort( pakfiles, numfiles, sizeof(char*), paksort );
+	if ( pakfiles ) {
+		qsort( pakfiles, numfiles, sizeof(char*), paksort );
+	}
 
 	if ( fs_numServerPaks ) {
 		numdirs = 0;