misc/swig/stdio: fix broken nil pointer test

misc/swig/stdio: fix broken nil pointer test

SWIG has always returned a typed interface value for a C++ class,
so the interface value will never be nil even if the pointer itself
is NULL. ptr == NULL in C/C++ should be ptr.Swigcptr() == 0 in Go.

Fixes #9514.

Change-Id: I3778b91acf54d2ff22d7427fbf2b6ec9b9ce3b43
Reviewed-on: https://go-review.googlesource.com/2440
Reviewed-by: Ian Lance Taylor <iant@golang.org>