[libmultipath] Fix overflow in circular queue

[libmultipath] Fix overflow in circular queue

Multipathd has a circular buffer where messages are stored prior to being
passed into the system logs. Under certain circumstances the circular buffer
fills up; and the intention appears to be that new messages are dropped until
space is freed in the buffer.

However sometimes the result of a full circular buffer is to overwrite the
earlier messages before they have been passed to the system buffer; resulting
in the loss of early messages and corruption of later messages.

Signed-off-by: Hannes Reinecke <hare@suse.de>