fs: ubifs: Need to check return for being an error pointer

The return value from alloc_super can be an error pointer so the error
check needs to detect this as well as checking the pointer for being
NULL.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>

fs/ubifs/super.c

@@ -2425,7 +2425,7 @@ retry:
 	if (!s) {
 		spin_unlock(&sb_lock);
 		s = alloc_super(type, flags);
-		if (!s)
+		if (IS_ERR_OR_NULL(s))
 			return ERR_PTR(-ENOMEM);
 #ifndef __UBOOT__
 		goto retry;