Fix GH-16592 msg_send() crashes when the type does not serialize as expected.

It is assumed that the serialization always had initialised its buffer
zend_string, but in the case of a typ

Fix GH-16592 msg_send() crashes when the type does not serialize as expected.

It is assumed that the serialization always had initialised its buffer
zend_string, but in the case of a type not serialising, it is null.

close GH-16599

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