Description:
------------
Observer current_observed_frame is unconditionally updated to prev_execute_data. However prev_execute_data may point to an unobserved function, causing current_observed_frame not to be updated until another observed function ends.
Thus current_observed_frame may point to some already left unobserved function, which may already have been overwritten.

Test script:
---------------
Installing observers on a and d,

<?php

ini_set("memory_limit", "20M");

function d() {} // observed

function c() { // gets prev_execute_data after end of d()
        d();
}

function b() {
        c();
}

function bailout(...$args) {
        array_map("str_repeat", ["\xFF"], [100000000]);
}

function a() { // observed (first_observed_frame)
        b();
        bailout(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15); // overwrite the vm_stack containing prev_execute_data
}

a();

Expected result:
----------------
No crash.

Actual result:
--------------
Crash, with current_observed_frame pointing to something not being a valid frame on the vm_stack:

(gdb) bt
#0  0x0000000012cd24c9 in zend_observer_fcall_end_all () at /usr/src/debug/php-8.0.10/Zend/zend_observer.c:235
#1  0x0000000012be059b in php_request_shutdown (dummy=dummy@entry=0x0) at /usr/src/debug/php-8.0.10/main/main.c:1777