ng_parse: IP address parsing in netgraph eating too many characters

Once the final component of the IP address has been parsed, the offset
on the input must not be advanced, as this would remove an unparsed
character from the input.

Submitted by: Markus Stoff
Reviewed by: donner
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D26489