present: dont crash if address searched backwards

For /start/,/end/, the /end/ address can wrap around and match somewhere
before /start/. Handle this case without crashing.

Fixes golang/go#7163.

Change-Id: I6fc3b5048e88728c9403a1d1e284ea8ada567a1e
Reviewed-on: https://go-review.googlesource.com/21182
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
1 file changed