mikey: do not request more data on proto violation

Return value "-1" means that more data is requested. The dissector
actually doesn't have any idea what to do with the data, so just return
the data dissected so far.

tcp stream 3182 from c05-http-reply-r1.pcap (SampleCaptures) was
incorrectly detected as MIKEY, it was actually HTTP...

Change-Id: Idca3d3e2f85e821df70436a675699b5834236f89
Reviewed-on: https://code.wireshark.org/review/2887
Reviewed-by: Evan Huus <eapache@gmail.com>