fix possible wrong cast to 32-bit unsigned

fix possible wrong cast to 32-bit unsigned

when calculation the max_offset the (unsigned) leads to a cast
to a 32-bit unsigned integer depending on the platform. as a result
we update the max_offset everytime when it grows beyond 2^32.
this leads to a wrong return max_offset value if the callbacks
are received out of order.

Signed-off-by: Peter Lieven <pl@kamp.de>