The general policy for the current state of js/wasm is that it only
has to support tests that are also supported by nacl.

The test nilptr3.go makes assumptions about which nil checks can be
removed. Since WebAssembly does not signal on reading a null pointer,
all nil checks have to be explicit.

Updates #18892

Change-Id: I06a687860b8d22ae26b1c391499c0f5183e4c485
Reviewed-on: https://go-review.googlesource.com/110096Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>