[Go] #2245 Handle NULL pointers for string* conversions.

Rearrange generation of director methods and rename
receiver argument from p to swig_p.

Fixes #2245

Examples/test-suite/go/go_director_inout_runme.go

@@ -67,6 +67,9 @@ func main() {
 
 	a.CallS4([]string{ "T1", "T2" })
 
+	a.S5(&str)
+	a.S5(nil)
+
 	a = wrap.NewDirectorMyClass(nil)
 	s = a.Adjust(m)
 	if s.Str != `{"first":"second"}` {