SYNOPSIS
#include <paradox.h>
void * PX_get_opaque(pxdoc_t *pxdoc)
DESCRIPTION
Calling the error handler will pass user data to it as specified
with PX_new3(3). The pointer to this user data
can be retrieve with this function.
RETURN VALUE
Returns pointer to user data on success and NULL on failure or if the user data is NULL.