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
Differential Revision: https://reviews.freebsd.org/D26489

(cherry picked from commit 63b6a08ce2467b8e230e7a4ecb3e1ddf1b48851c)