database/sql: add missing unlock when context is expired

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>

src/database/sql/sql.go

@@ -814,6 +814,7 @@ func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn
 	}
 	// Check if the context is expired.
 	if err := ctx.Err(); err != nil {
+		db.mu.Unlock()
 		return nil, err
 	}
 	lifetime := db.maxLifetime