Fixed crash on attempt to load corrupted data file

Instance of FileReader is no longer destructed when header of data file is valid but complete loading of file fails
Loading of truncated .zip file is the simplest test case

src/resourcefiles/file_zip.cpp

@@ -423,6 +423,8 @@ FResourceFile *CheckZip(const char *filename, FileReader *file, bool quiet)
 		{
 			FResourceFile *rf = new FZipFile(filename, file);
 			if (rf->Open(quiet)) return rf;
+
+			rf->Reader = NULL; // to avoid destruction of reader
 			delete rf;
 		}
 	}