[release-branch.go1.10] database/sql: check for nil connRequest.conn before use

The connRequest may return a nil conn value. However in a rare
case that is difficult to test for it was being passed to
DB.putConn without a nil check. This was an error as this
made no sense if the driverConn is nil. This also caused
a panic in putConn.

A test for this would be nice, but didn't find a sane
way to test for this condition.

Updates #24445
Fixes #25235

Change-Id: I827316e856788a5a3ced913f129bb5869b7bcf68
Reviewed-on: https://go-review.googlesource.com/102477


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexey Palazhchenko <alexey.palazhchenko@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit b98ffdf8)
Reviewed-on: https://go-review.googlesource.com/c/146778


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>