The prefix computation for onepass was incorrectly returning
complete=true when it encountered a beginning-of-text empty width match
(^) in the middle of an expression.

Fix by returning complete only when the prefix is followed by $ and then
an accepting state.

Fixes #11175.

Change-Id: Ie9c4cf5f76c1d2c904a6fb2f016cedb265c19fde
Reviewed-on: https://go-review.googlesource.com/16200
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>