test: use a real use function in nilptr2.go
test: use a real use function in nilptr2.go

Adjust the dummy use function to a real use. As suggested by the
println calls in the test, nilptr2.go supposes to check that a
used nil pointer dereference panics. This use function is not
real enough so an optimized compiler such as gccgo could
eliminate the call.

The spec requires that even a dummy use would cause a panic.
Unfortunately, due to #31151 this is not true for gccgo at -O1 or
above.

Change-Id: Ie07c8a5969ab94dad82d4f7cfec30597c25b7c46
Reviewed-on: https://go-review.googlesource.com/c/go/+/176579
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,golang/go,cloudflare/go,akutz/go,akutz/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go