fmt: put back named results in ss.scanBasePrefix

CL 165619 removed these names when it removed
the use of the plain 'return'. But the names help for
documentation purposes even without being mentioned
directly in the function, so removing them makes the
code less readable. Put them back. I renamed found
to zeroFound to make the meaning clearer.

Change-Id: I1010931f08290af0b0ede7d21b1404c2eea196a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/165899


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>