builtin-fetch: Don't segfault on "fetch +foo"

builtin-fetch: Don't segfault on "fetch +foo"

If we are fetching something and were configured to do a forced
fetch and have no local ref to store the fetched object into we
cannot mark the local ref as having a forced update.  Instead we
should just silently discard the + request.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>