VoIP calls: Use our tap offsets.

Add convenience functions to convert between tap IDs and tapinfo
pointers. Use them to get rid of some calls to voip_calls_get_info().

Lowercase function names. Remove more exit()s.

Change-Id: I187fdc505bc22cc05edafed9e97ede8279ea53f9
Reviewed-on: https://code.wireshark.org/review/5321
Reviewed-by: Gerald Combs <gerald@wireshark.org>