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/2440Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/swig/stdio/file_test.go

@@ -10,7 +10,7 @@ import "testing"
 // as expected.
 func TestRead(t *testing.T) {
 	f := Fopen("file_test.go", "r")
-	if f == nil {
+	if f.Swigcptr() == 0 {
 		t.Fatal("fopen failed")
 	}
 	if Fgetc(f) != '/' || Fgetc(f) != '/' || Fgetc(f) != ' ' || Fgetc(f) != 'C' {