cmd/gofmt: clear pattern match map at the correct time

cmd/gofmt: clear pattern match map at the correct time

We need to clear the pattern match map after the recursive rewrite
applications, otherwise there might be lingering entries that cause
match to fail.

Fixes #18987.

Change-Id: I7913951c455c98932bda790861db6a860ebad032
Reviewed-on: https://go-review.googlesource.com/36546
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>