only try to find a common folder in Zips if there's some data to chec…

…k against.

Without this the check for required file names will always fail so running the entire thing is pointless.

src/common/filesystem/source/resourcefile.cpp

@@ -383,8 +383,8 @@ void FResourceFile::PostProcessArchive(LumpFilterInfo *filter)
 
 	// Entries in archives are sorted alphabetically.
 	qsort(Entries, NumLumps, sizeof(Entries[0]), entrycmp);
-	FindCommonFolder(filter);
 	if (!filter) return;
+	FindCommonFolder(filter);
 
 	// Filter out lumps using the same names as the Autoload.* sections
 	// in the ini file. We reduce the maximum lump concidered after