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

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>