bugfix for vanilla diablo (set archive size to zero in case the file did not exists)

Source/mpqapi.cpp

@@ -207,6 +207,7 @@ struct Archive {
 			}
 		} else {
 			mode |= std::ios::trunc;
+			this->size = 0;
 		}
 		if (!stream.Open(name, mode)) {
 			stream.Close();