Fix memory leak when setting an invalid DOMDocument encoding

Because the failure path did not release the string, there was a memory
leak.
As the only valid types for this function a

Fix memory leak when setting an invalid DOMDocument encoding

Because the failure path did not release the string, there was a memory
leak.
As the only valid types for this function are IS_NULL and IS_STRING, we
and IS_NULL is always rejected in practice, solve the issue by not using
a function that increments the refcount in the first place.

Closes GH-12002.

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