fix a crash with failure to open second file for dual-file stereo

git-svn-id: https://vgmstream.svn.sourceforge.net/svnroot/vgmstream@258 51a99a44-fe44-0410-b1ba-c3e57ba2b86b

src/vgmstream.c

@@ -957,7 +957,8 @@ void try_dual_file_stereo(VGMSTREAM * opened_stream, STREAMFILE *streamFile) {
 #endif
 
     dual_stream = streamFile->open(streamFile,filename2,STREAMFILE_DEFAULT_BUFFER_SIZE);
-    // no need to check NULL here since init_vgmstream_internal does it at its beginning
+    if (!dual_stream) goto fail;
+
     new_stream = init_vgmstream_internal(dual_stream,
             0   /* don't do dual file on this, to prevent recursion */
             );