script/execute: Fix potential underflow and NULL dereference

The result is initialized to 0 in grub_script_arglist_to_argv().
If the for loop condition is not met both result.args and result.argc
remain 0 causing result.argc - 1 to underflow and/or result.args NULL
dereference. Fix the issues by adding relevant checks.

Fixes: CID 473880

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/script/execute.c

@@ -760,6 +760,9 @@ cleanup:
 	}
     }
 
+  if (result.args == NULL || result.argc == 0)
+    goto fail;
+
   if (! result.args[result.argc - 1])
     result.argc--;