Fix the very long loop reported in https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=8911 :
In parseFields() ensure that we have at least one byte so that callers
don't have to protect against it returning an offset which hasn't incremented.
Remove a couple of now-unnecessary length-remaining (aka "will the offset
In some other checks, use tvb_ensure_length_remaining() rather than calling
tvb_length_remaining() and (potentially) THROWing an exception. I'm not sure
if these are really necessary now or not...