cmd/cgo: compare malloc return value to NULL instead of literal 0

cmd/cgo: compare malloc return value to NULL instead of literal 0

Follow the pattern used in the other _cgoPREFIX_Cfunc* functions. This
also avoids a -Wzero-as-null-pointer-constant warning when compiling
with g++.

Change-Id: I95ac8842df048105f4c738f3603136d9cfa2dfdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/664916
Auto-Submit: Keith Randall <khr@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>