Fix: File browser "asset" context query not ensuring file is asset

The "asset" context query would just always return the active file (if any),
without ensuring that it's actually an asset. Mistake in 3f9c943243.

source/blender/editors/space_file/file_context.cc

@@ -83,7 +83,7 @@ int /*eContextResult*/ file_context(const bContext *C,
 
   if (CTX_data_equals(member, "asset")) {
     FileDirEntry *file = filelist_file(sfile->files, params->active_file);
-    if (file == nullptr) {
+    if (file == nullptr || !file->asset) {
       return CTX_RESULT_NO_DATA;
     }