Treat a length of -1 as a Very Large Length in most cases.

For field types where it doesn't mean "to the end of the tvbuff", treat
it like other negative length values - as if it were unsigned and thus
Very Large and thus likely to be past the end of the tvbuff.  That way,
some of the "we hax0red your SNMP packets" captures, with length fields
of 0xffffffff, report malformed packets rather than dissector bugs.

Change-Id: Id53f828b06b6febe7d79f8539e54523e0b43e5c2
Reviewed-on: https://code.wireshark.org/review/4091
Reviewed-by: Guy Harris <guy@alum.mit.edu>