rework some of the friend-dependent access to FResourceLump.

src/common/filesystem/source/file_pak.cpp

@@ -138,9 +138,7 @@ FResourceFile *CheckPak(const char *filename, FileReader &file, LumpFilterInfo*
 		{
 			auto rf = new FPakFile(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;