ay_to_strv: Prevent dereferencing a NULL pointer.

* clang static analysis shows that ay_to_strv is
  called with argc set to NULL. This can cause
  ay_to_strv to dereference the pointer in a
  failure situation.

src/terminal.c

@@ -82,7 +82,8 @@ ay_to_strv (GVariant *variant,
 	data = g_variant_get_fixed_array (variant, &data_len, sizeof (char));
 	if (data_len == 0 || data_len > G_MAXSSIZE)
 	{
-		*argc = 0;
+		if (argc != NULL)
+			*argc = 0;
 		return NULL;
 	}