apparmor: Fix aa_label_build() error handling for failed merges

aa_label_merge() can return NULL for memory allocations failures
make sure to handle and set the correct error in this case.

Reported-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/domain.c

@@ -1444,7 +1444,10 @@ check:
 			new = aa_label_merge(label, target, GFP_KERNEL);
 		if (IS_ERR_OR_NULL(new)) {
 			info = "failed to build target label";
-			error = PTR_ERR(new);
+			if (!new)
+				error = -ENOMEM;
+			else
+				error = PTR_ERR(new);
 			new = NULL;
 			perms.allow = 0;
 			goto audit;