Protect against segfaults calling destroyed closures

[alexcrichton]

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

[takkuumi]

@alexcrichton

But now, there was another bug.

Error: closure invoked recursively or destroyed already
    at module.exports.__wbindgen_throw (/Users/ling/Codelab/tswasm/rust_assembly/pkg/rust_assembly.js:390:11)
    at wasm_bindgen::throw_str::hda700a983b9b66a8 (wasm-function[121]:5)
    at <dyn +core::ops::function::FnMut<()>+Output = R as wasm_bindgen::closure::WasmClosure>::describe::invoke::h72c772442484abe4 (wasm-function[51]:65)
    at Function.cb (/Users/ling/Codelab/tswasm/rust_assembly/pkg/rust_assembly.js:401:20)
    at listOnTimeout (internal/timers.js:531:17)
    at processTimers (internal/timers.js:475:7)

for issue RuntimeError: memory access out of bounds #1526

[alexcrichton]

@NateLing I believe that is a legitimate bug in that branch (if it's the same as I remember) which is JS calling a destroyed closure.