Fix a missing early-return in Table::get (#1652)

Turns out this was a typo from #1016!

tests/all/table.rs

@@ -0,0 +1,13 @@
+use wasmtime::*;
+
+#[test]
+fn get_none() {
+    let store = Store::default();
+    let ty = TableType::new(ValType::FuncRef, Limits::new(1, None));
+    let table = Table::new(&store, ty, Val::AnyRef(AnyRef::Null)).unwrap();
+    match table.get(0) {
+        Some(Val::AnyRef(AnyRef::Null)) => {}
+        _ => panic!(),
+    }
+    assert!(table.get(1).is_none());
+}