dm: test: Don't clear global_data in dm_test_uclass_before_ready()

We must not clear global_data even in tests, since the ram_buffer (which
is used by malloc()) will also be lost, and subsequent tests will fail.

Zero only the global_data fields that are required for the test to function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Tested-by: Joe Hershberger <joe.hershberger@ni.com>

test/dm/core.c

@@ -651,7 +651,10 @@ static int dm_test_uclass_before_ready(struct dm_test_state *dms)
 
 	ut_assertok(uclass_get(UCLASS_TEST, &uc));
 
-	memset(gd, '\0', sizeof(*gd));
+	gd->dm_root = NULL;
+	gd->dm_root_f = NULL;
+	memset(&gd->uclass_root, '\0', sizeof(gd->uclass_root));
+
 	ut_asserteq_ptr(NULL, uclass_find(UCLASS_TEST));
 
 	return 0;