[release-branch.go1.8] database/sql: ensure releaseConn is defined before a possible...

[release-branch.go1.8] database/sql: ensure releaseConn is defined before a possible close

Applies https://golang.org/cl/42139 to the go1.8 release branch.

Also correct two minor issues detected with go vet.

Fixes #20217

Change-Id: I2c41af9497493598fbcfc140439b4e25b9bb7e72
Reviewed-on: https://go-review.googlesource.com/42532
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>