bpo-44285: getpath.c: Assert that env_file is NULL during an error check (GH-26486)

This was flagged by a static analyzer, but the logic of why this is NULL on error is hard to follow for humans as well.

Modules/getpath.c

@@ -1263,6 +1263,7 @@ calculate_read_pyenv(PyCalculatePath *calculate)
 
     status = calculate_open_pyenv(calculate, &env_file);
     if (_PyStatus_EXCEPTION(status)) {
+        assert(env_file == NULL);
         return status;
     }
     if (env_file == NULL) {