zend_std_get_property_ptr_ptr() was the only property handler that did
not propagate the IN_GET guard to the underlying object when forwarding
from a lazy proxy after initialization. This caused __get to be called
on the underlying object when it shouldn't be, leading to assertion
failures.
The same guard-copying pattern already existed in read_property,
write_property, unset_property, and has_property since commit
26f5009e91 (GH-18039).
Also fixes GH-20873 and GH-20854.
Closes GH-20875
