amqp: ref the LogMessage before pushing it to the worker queue

If the LogMessage is not ref()d, it will be free()d up before the worker
thread pops it out of the queue, leading to severe problems. Ref it like
all other drivers do.

Reported-by: Alexandre Biancalana <biancalana@gmail.com>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>