reassembly: fix when fragment cut off by snapshot

Also add an assertion to tvb_generic_clone_offset_len so that it throws an
error *before* allocating memory, as otherwise that memory is leaked.

Bug: 10474
Change-Id: I5036cefac16841914a59670c64979cf599bf7969
Reviewed-on: https://code.wireshark.org/review/4234
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>