Methods of FileSet are documented to be safe for concurrent use by
multiple goroutines, so FileSet is protected by a mutex and all its
methods use it to prevent concurrent mutations. All methods of File that
mutate the respective FileSet, including AddLine, do also lock its
mutex, but that does not help when PositionFor is invoked concurrently
and reads without synchronization what AddLine mutates.

The change adds acquiring a RLock around the racy call of File.position
and the respective test.

Fixes #16548

Change-Id: Iecaaa02630b2532cb29ab555376633ee862315dd
Reviewed-on: https://go-review.googlesource.com/25345Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>