normal/completion: Add missing grub_malloc() failure check

This patch adds a NULL check after grub_malloc() call. Missing a failure
check after calling grub_malloc() can lead to undefined behavior. If the
allocation fails and returns NULL subsequent dereferencing or writing to
the pointer will likely result in a runtime error such as a segmentation
fault.

Signed-off-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/normal/completion.c

@@ -490,6 +490,9 @@ grub_normal_do_completion (char *buf, int *restore,
 	  spaces++;
 
       ret = grub_malloc (match_len - current_len + grub_strlen (suffix) + spaces + 1);
+      if (ret == NULL)
+        goto fail;
+
       newstr = ret;
       for (escstr = match + current_len; *escstr; escstr++)
 	{