Fix the uninitialized-read error reported in bug 10370.

Fix apparent cut-n-pasteo: if offset_e is set then we should be looking
at offset_e (instead of offset_s) and ts_end (instead of ts_start).

Bug: 10370
Change-Id: I0683ad55ac782c7fd03b7dbdf3f34df713267de5
Reviewed-on: https://code.wireshark.org/review/3589
Reviewed-by: Evan Huus <eapache@gmail.com>