Don't use tvb_bytes_exist() to check if we captured the whole frame, just
compare captured_length to reported_length (which we've already retrieved
anyway).

Replace calls to tvb_length() with non-deprecated versions.

Change-Id: I9f6f3461915573fb21716a87cb6af416a3656097
Reviewed-on: https://code.wireshark.org/review/4080
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>