Fix GH-15181: Disabled output handler is flushed again

When an `PHP_OUTPUT_HANDLER_FAILURE` occurs, the output handler becomes
disabled (i.e. the `PHP_OUTPUT_HANDLER_DISABLED` flag is se

Fix GH-15181: Disabled output handler is flushed again

When an `PHP_OUTPUT_HANDLER_FAILURE` occurs, the output handler becomes
disabled (i.e. the `PHP_OUTPUT_HANDLER_DISABLED` flag is set). However,
there is no guard for disabled handlers in `php_output_handler_op()`
what may cause serious issues (as reported, UB due to passing `NULL` as
the 2nd argument of `memcpy`, because the handler's buffer has already
been `NULL`ed). Therefore, we add a respective guard for disabled
handlers, and return `PHP_OUTPUT_HANDLER_FAILURE` right away.

Closes GH-15183.

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