tvbuff: make tvb_bytes_exist fail with negative values

tvbuff: make tvb_bytes_exist fail with negative values

tvb_generic_clone_offset_len uses tvb_bytes_exist to check that the
requested tvb data is actually available. It did not expect negative
values, that would result in an overly large memory allocation.

Bug: 14678
Change-Id: Ie80095a381e55ca5dbbd5c9d835243549d0b212e
Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7179
Reviewed-on: https://code.wireshark.org/review/27526
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Anders Broman <a.broman58@gmail.com>