Fix assertion failure in zend_std_read_property

[arnaud-lb]

Fixes GH-16615.

We assert that Z_PROP_FLAG_P(retval) is exactly IS_PROP_UNINIT, but this is a bit field and it may contain irrelevant bits. For instance, props may also have the IS_PROP_LAZY or IS_PROP_REINITABLE bits, which does not change the fact that the property is uninitialized:

<?php

class Foo {
    public string $bar {
        set => $value;
    }
    public function __clone() {
        // Assertion failure
        echo $this->bar;
    }
}

clone new Foo();

@nielsdos I didn't update the comment because it's still correct, and I think that using equality comparison was unintended.

[iluuu1994]

Ah, silly mistake on my part. Thank you!