database/sql: check to see if ctx is cancelable before await

Prevent queries from starting a goroutine if the context is
not able to be canceled.

Fixes #23879

Change-Id: I392047bd53d7f796219dd12ee11b07303658fdaf
Reviewed-on: https://go-review.googlesource.com/102478
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -2563,6 +2563,9 @@ type Rows struct {
 }
 
 func (rs *Rows) initContextClose(ctx, txctx context.Context) {
+	if ctx.Done() == nil && (txctx == nil || txctx.Done() == nil) {
+		return
+	}
 	ctx, rs.cancel = context.WithCancel(ctx)
 	go rs.awaitDone(ctx, txctx)
 }