dont't manually throw a ReportedBoundsError if the packet is empty
use tvb_captured_length() instead of tvb_length()

Change-Id: I7e7efd69515fc3e30c986ac5d9a56b4db1931c10
Reviewed-on: https://code.wireshark.org/review/5624
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>