libceph: skip message if too big to receive

We know the length of our message buffers.  If we get a message
that's too long, just dump it and ignore it.  If skip was set
then con->in_msg won't be valid, so be careful not to dereference
a null pointer in the process.

This resolves:
    http://tracker.ceph.com/issues/4664



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>