RADIUS client: Fix previous failover change

RADIUS client: Fix previous failover change

Commit 347c55e216f22002246e378097a16ecb24b7c106 ('RADIUS client: Re-try
connection if socket is closed on retransmit') added a possibility of
executing RADIUS server failover change within
radius_client_retransmit() without taking into account that this
operation may end up freeing the pending message that is being
processed. This could result in use of freed memory. Avoid this by
checking whether any pending messages have been removed and if so, do
not try to retransmit the potentially freed message.

Signed-off-by: Jouni Malinen <j@w1.fi>