dbus_new_handlers: Don't send NULL to dbus_message_new_error

The new DBus API helper function wpas_dbus_error_unknown_error
function can be called as a result of a failure within internal
getter calls, which will call this function with a NULL message
parameter.  However, dbus_message_new_error looks very unkindly
(i.e, abort()) on a NULL message, so in this case, we should not
call it.

I've observed this course of events during a call to
wpas_dbus_getter_bss_wpa with a faileld parse of the IE parameter.
We got here through a call to fill_dict_with_properties which
explicitly calls getters with a NULL message parameter.  Judging
from the way it is called, this could easily occur if an AP sends
out a malformed (or mis-received) probe response.  I usually run
into this problem while driving through San Francisco, so I'm
exposed to any number of base stations along this path.

wpa_supplicant/dbus/dbus_new_handlers.c

@@ -117,6 +117,20 @@ static char * wpas_dbus_new_decompose_object_path(const char *path,
 DBusMessage * wpas_dbus_error_unknown_error(DBusMessage *message,
 					    const char *arg)
 {
+	/*
+	 * This function can be called as a result of a failure
+	 * within internal getter calls, which will call this function
+	 * with a NULL message parameter.  However, dbus_message_new_error
+	 * looks very unkindly (i.e, abort()) on a NULL message, so
+	 * in this case, we should not call it.
+	 */
+	if (message == NULL) {
+		wpa_printf(MSG_INFO, "dbus: wpas_dbus_error_unknown_error "
+			   "called with NULL message (arg=%s)",
+			   arg ? arg : "N/A");
+		return NULL;
+	}
+
 	return dbus_message_new_error(message, WPAS_DBUS_ERROR_UNKNOWN_ERROR,
 				      arg);
 }