Protect against segfaults calling destroyed closures

This commit updates the drop glue generated for closures to simply
ignore null pointers. The drop glue can be called in erroneous
situations such as when a closure is invoked after it's been destroyed.
In these cases we don't want to segfault and/or corrupt memory but
instead let the normal error message from the invoke glue continue to
get propagated.

Closes #1526

tests/wasm/closures.js

@@ -119,3 +119,7 @@ exports.pass_reference_first_arg_twice = (a, b, c) => {
   c(a);
   a.free();
 };
+
+exports.call_destroyed = f => {
+  assert.throws(f, /invoked recursively or destroyed/);
+};