This is a 2nd attempt at fixing CL 162337 which had an off-by-one error.
We were unconditionally getting the position of the start of the next line
from the last import without checking whether it is the end of the file or not.

Fix the code to check for that and move the testcase added in CL 190523
to the end of the file for it to trigger the issue properly.

Fixes #18929

Change-Id: I59e77256e256570b160fea6a17bce9ef49e810df
Reviewed-on: https://go-review.googlesource.com/c/go/+/190480
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>