Fixed 2 minor bugs in JS glue code

[RunDevelopment]

This separates the 2 bug fixes from #4189 into their own PR.

Changes:

1. debugString (this is the JS glue code version of format!("{:?}", value)) had a bug where it checked for a regex mismatch incorrectly. This would lead to dereferencing null at runtime, which throws a TypeError.

2. _assertClass returned instance.ptr. Here's the full code of the function:

   function _assertClass(instance, klass) {
       if (!(instance instanceof klass)) {
           throw new Error(`expected instance of ${klass.name}`);
       }
       return instance.ptr;
   }

   Returning instance.ptr is weird for 2 reasons:

   • It's not used anywhere in the code. _assertClass is only used in one place and its return value is ignored. As @daxpedda found out, the return value hasn't been used for 6 years now.
   • There is no ptr field. The ptr field was renamed to __wbg_ptr one year ago and this function wasn't updated.

   So I just removed the return statement and instance.ptr.

[daxpedda]

Thank you!
LGTM.

But missing a changelog entry for the regex issue.