Trying to create and use an empty tvb now throws an exception.  Just pass current tvb to mac-lte dissector.

Change-Id: I4aa24342192ec079254f26e00317470377c3e20b
Reviewed-on: https://code.wireshark.org/review/2916
Reviewed-by: Martin Mathieson <martin.r.mathieson@googlemail.com>