Fix edge case serializing __PHP_Incomplete_Class properties.

This was using strcmp instead of zend_string_equals_literal.
As a result, the property count didn't match the number of prope

Fix edge case serializing __PHP_Incomplete_Class properties.

This was using strcmp instead of zend_string_equals_literal.
As a result, the property count didn't match the number of properties
being serialized if properties started with
"__PHP_Incomplete_Class\0" (unlikely)

(before, `'O:8:"Missing_":1:{}'` would be serialized, which failed to
unserialize)

Everywhere else expects the MAGIC_MEMBER to match exactly,
and this should use zend_string_equals_literal as an example for other code.

This has used strcmp since 2004 in deb84befae4bbc3686a4f2ed82b04e2cabae5dc0

Closes GH-6555

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