Support both decimal and hex values for uint dissectors.

And, while we're at it, reject entries with invalid numbers.

Change-Id: Ifefe28396cfe391d2d5c7f7ff335d0f041e35a03
Reviewed-on: https://code.wireshark.org/review/3417
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit 01361a92cbb53d229fe927b9765dcd6c7999efb4)
Reviewed-on: https://code.wireshark.org/review/3418