Fix codegen of consuming setters/getters (#2172)

Make sure they reset their internal pointer to null after we call Rust
since it invalidates the Rust pointer after being called!

Closes #2168

examples/add/src/lib.rs

@@ -4,3 +4,21 @@ use wasm_bindgen::prelude::*;
 pub fn add(a: u32, b: u32) -> u32 {
     a + b
 }
+
+#[wasm_bindgen]
+#[derive(Copy, Clone)]
+pub struct Answer(u32);
+
+#[wasm_bindgen]
+impl Answer {
+    pub fn new() -> Answer {
+        Answer(41)
+    }
+    #[wasm_bindgen(getter)]
+    pub fn the_answer(self) -> u32 {
+        self.0 + 1
+    }
+    pub fn foo(self) -> u32 {
+        self.0 + 1
+    }
+}