User registered opcode handler should call ones already set by other extensions

[derickr]

This extension registers an opcode handler. This handler currently returns the ZEND_USER_OPCODE_DISPATCH value which means that the Zend engine users its internal implementation to further handle that opcode. If however another extension (such as Xdebug) has also overridden the opcode, its handler will not be called.

Make sure to remember already set handlers, and then call these if they're not NULL, and otherwise continue returning ZEND_USER_OPCODE_DISPATCH.

[derickr]

See also https://bugs.xdebug.org/1759