Missing the DB mutex unlock on an early return after checking
if the context has expired.

Fixes #17518

Change-Id: I247cafcef62623d813f534a941f3d5a3744f0738
Reviewed-on: https://go-review.googlesource.com/31494Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>