Minor performance tweaks:
- Only calculate tvb length once.
- Use tvb_reported_length() instead of tvb_reported_length_remaining() as
  this is a subtvb offset is always 0.

Change-Id: I03bd7a95061488d4576fa93f26e6b31d55f88738
Reviewed-on: https://code.wireshark.org/review/4060
Reviewed-by: Anders Broman <a.broman58@gmail.com>