[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>

src/database/sql/sql.go

@@ -1955,12 +1955,12 @@ func (s *Stmt) QueryContext(ctx context.Context, args ...interface{}) (*Rows, er
 				rowsi: rowsi,
 				// releaseConn set below
 			}
-			rows.initContextClose(ctx)
 			s.db.addDep(s, rows)
 			rows.releaseConn = func(err error) {
 				releaseConn(err)
 				s.db.removeDep(s, rows)
 			}
+			rows.initContextClose(ctx)
 			return rows, nil
 		}
 

src/database/sql/sql_test.go

@@ -322,7 +322,7 @@ func TestQueryContext(t *testing.T) {
 	select {
 	case <-ctx.Done():
 		if err := ctx.Err(); err != context.Canceled {
-			t.Fatalf("context err = %v; want context.Canceled")
+			t.Fatalf("context err = %v; want context.Canceled", ctx.Err())
 		}
 	default:
 		t.Fatalf("context err = nil; want context.Canceled")
@@ -413,7 +413,8 @@ func TestTxContextWait(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)
 
-	ctx, _ := context.WithTimeout(context.Background(), time.Millisecond*15)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*15)
+	defer cancel()
 
 	tx, err := db.BeginTx(ctx, nil)
 	if err != nil {