script/execute: Add missing grub_strdup() failure check

If grub_strdup() fails, it returns NULL and passing NULL further down to
the code can lead to segmentation fault or an undefined behavior.

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/script/execute.c

@@ -1015,6 +1015,9 @@ grub_script_execute_cmdline (struct grub_script_cmd *cmd)
 	{
 	  /* As a last resort, try if it is an assignment.  */
 	  char *assign = grub_strdup (cmdname);
+          if (assign == NULL)
+            return grub_errno;
+
 	  char *eq = grub_strchr (assign, '=');
 
 	  if (eq)