Bugfix: Ensure previous fiber context can be destroyed in fiber trampoline

[kooldev]

This fixes a memory leak that occurs when symmetric coroutines ares used like this:

await([
	async(fn() => 1),
	async(fn() => 2)
]);

In this case control will be transfered from main to fiber 1 to fiber 2 and back to main. Without this PR this code will result in a memory leak (fiber 1 cannot be released). The fiber context of fiber 1 has to be destroyed immediately after switching to fiber 2 because it is already marked as dead and will never be resumed. The context of fiber 2 is destroyed when main is resumed. Adding additional fibers in the example will fail to destroy every fiber context except for the last one.

@trowski I discovered this bug due to zend_fiber_stack being a tracked allocation after your last commit. 😉

[trowski]

Good find, that's a subtle bug (that is, before implementing a symmetric coroutine 😆). I'll add a test for it in zend_test.