strutil: SkipUntil: restore function and limit outcome to len
strutil: SkipUntil: restore function and limit outcome to len

In case we are dealing with a string that is not terminated after len,
limit the result of SkipUntil to either a matching character or the
given length. Whatever is minimal.

Signed-off-by: Matthias Maennich <9bce818340fa3fdb97f10fe3386d272c8064360e@google.com>

google/kati,google/kati,google/kati,google/kati,google/kati