[wasm] Fix Asyncify loop exit condition for normal return

Stop calling `asyncify_stop_unwind` when the main function returns
without any unwinding. In the era when Asyncify buffers were allocated
on the stack, the `top` and `end` fields were remained in the stack
space even after the main function returned, so buffer-overflow check in
the `asyncify_stop_unwind` function passed. But now, the `top` and `end`
fields are part of the jump buffer allocated on the heap and they are
deallocated with `free` when the corresponding VM tag is popped. So, the
buffer-overflow check in the `asyncify_stop_unwind` function failed when
the main fuction returned without any unwinding, and we have to break
the asyncify loop before calling `asyncify_stop_unwind`.

Related commit: https://github.com/ruby/ruby.wasm/commit/bc46b12b127e4b6625a100f373504ed5ea45ae66