Fix crash when no callback is provided

mpi/src/starpu_mpi_collective.c

@@ -138,12 +138,12 @@ int starpu_mpi_gather_detached(starpu_data_handle_t *data_handles, int count, in
 				}
 			}
 		}
-	}
 
-	if (!callback_arg->count)
-	{
-		free(callback_arg);
-		return 0;
+		if (!callback_arg->count)
+		{
+			free(callback_arg);
+			return 0;
+		}
 	}
 
 	for(x = 0; x < count ; x++)