When replicating an update pair (before image, after image) under row-based
replication, and the before image is not found on the slave, the after image
was not discared, and was hence read as a before image for the next row.
Eventually, this lead to an after image being read outside the block of rows
in the event, causing an assertion to fire.

This patch fixes this by reading the after image in the event that the row
was not found on the slave, adds some extra debug assertion to catch future
errors earlier, and also adds a few non-debug checks to prevent reading
outside the block of the event.