This avoids a possible significant performance penalty, when some leaf function was observed, deep in the stack.
As a side effect, we are not iterating over prev_execute_data anymore and thus, non-observed fake frames, possibly on stack, cannot have any impact on the observer anymore (especially within zend_observer_fcall_end_all).
Saving the previous observer happens now directly on the VM stack. If there is any observer, function frames are allocated an extra zval (the last temporary), which will, on observed frames, contain the previous observed frame address.
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