CLEANUP: pattern: ensure that payload and payload_lv always stay in the buffer

CLEANUP: pattern: ensure that payload and payload_lv always stay in the buffer

A test was already performed which worked by pure luck due to integer types,
otherwise it would have been possible to start checking for an offset out of
the buffer's bounds if the buffer size was large enough to allow an integer
wrap. Let's perform explicit checks and use unsigned ints for offsets instead
of risking being hit later.