Fix #81435 Observer current_observed_frame may point to an old (overwritten) frame

Ensure current_observed_frame always points to an actually observed frame.
This solution has a caveat o

Fix #81435 Observer current_observed_frame may point to an old (overwritten) frame

Ensure current_observed_frame always points to an actually observed frame.
This solution has a caveat of being O(stack size), with the worst case occurring if there are a lot of frames between the current and previous observed frames.
An O(1) solution would require keeping track of the previous observed frame, which would require some additional frame attached metadata, which is best not attempted in an already released version.

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XFAIL observer_error_05.phpt test

Skip dummy frames allocated on CPU stack of zend_call_function().
(Usage of "current_observed_frame" varible looks unsafe to me).