cgen: fix an error when interpo a ptr and specifier is p(fix #19048)

[shove70]

1. Fix Unable to print memory address of a pointer receiver #19048
2. Add test.

struct Foo {
}

fn (mut f Foo) intp_pointer() string {
	return '${f:p}'
}

fn main() {
	mut foo := Foo{}
	str1 := foo.intp_pointer()

	str2 := '${&foo:p}'
	assert str1 == str2
}

passed

[spytheman]

Good work.