[release-branch.go1.10] cmd/cover: don't crash on non-gofmt'ed input

Without the change to cover.go, the new test fails with

panic: overlapping edits: [4946,4950)->"", [4947,4947)->"thisNameMustBeVeryLongToCauseOverflowOfCounterIncrementStatementOntoNextLineForTest.Count[112]++;"

The original code inserts "else{", deletes "else", and then positions
a new block just after the "}" that must come before the "else".
That works on gofmt'ed code, but fails if the code looks like "}else".
When there is no space between the "{" and the "else", the new block
is inserted into a location that we are deleting, leading to the
"overlapping edits" mentioned above.

This CL fixes this case by not deleting the "else" but just using the
one that is already there. That requires adjust the block offset to
come after the "{" that we insert.

Fixes #23927

Change-Id: I40ef592490878765bbce6550ddb439e43ac525b2
Reviewed-on: https://go-review.googlesource.com/98935


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-on: https://go-review.googlesource.com/102786


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>