When existing data in buffer does not have delimiter,
and new data is added with b.fill(), continue search from
previous point instead of starting from beginning.

Change-Id: Id78332afe2b0281b4a3c86bd1ffe9449cfea7848
GitHub-Last-Rev: 08e7d2f50151a00b22800e3f7020d0de8dee7dcf
GitHub-Pull-Request: golang/go#25441
Reviewed-on: https://go-review.googlesource.com/113535
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>