avm2: Don't hold GcCell read ref open for Loader.loadBytes

[Aaron1011]

We were holding this open while dispatching an event, leading to a panic if anything attempted to modify the ByteArray.

[Lord-McSweeney]

Can you add a test?

[Aaron1011]

I updated an existing test to print out bytearray.position from event handlers.

[Lord-McSweeney]

Are you sure that the test panics on current master? #14789 was a result of a mutable borrow (readUnsignedInt or whatever it was needs to update position) after the immutable borrow in the load_bytes, but the added test only calls bytearray::get_position, which doesn't mutably borrow the bytearray.

[torokati44]

I had the same concerns, so I downloaded the updated test SWF, and started it in the desktop player and the web demo, got an error in both.

[Aaron1011]

Yes, I confirmed that this errors on master. bytearray.position ends up mutating bytearray in order to attach the bound method.