rework some of the friend-dependent access to FResourceLump.

src/common/filesystem/source/file_whres.cpp

@@ -136,8 +136,7 @@ FResourceFile *CheckWHRes(const char *filename, FileReader &file, LumpFilterInfo
 		}
 		auto rf = new FWHResFile(filename, file, sp);
 		if (rf->Open(filter)) return rf;
-		file = std::move(rf->Reader); // to avoid destruction of reader
-		delete rf;
+		file = rf->Destroy();
 	}
 	return NULL;
 }