cmd/cgo: check pointers for deferred C calls at the right time

cmd/cgo: check pointers for deferred C calls at the right time

We used to check time at the point of the defer statement. This change
fixes cgo to check them when the deferred function is executed.

Fixes #15921.

Change-Id: I72a10e26373cad6ad092773e9ebec4add29b9561
Reviewed-on: https://go-review.googlesource.com/23650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>