HPROF: bad assert check on GetObjectMonitorUsage() returning usage.owner==null

Looks like hprof will assert if when using the JVM TI GetObjectMonitorUsage() returns usage.owner == NULL, but this could happen and is a valid return.

hprof_tls.c around line 616:

                io_write_monitor_dump_state(sig,
                       get_serial_number(env, usage.owner), <------
                       usage.entry_count,
                       waiter_nums, usage.waiter_count,
                       notify_waiter_nums, usage.notify_waiter_count);

Needs to allow for usage.owner to be NULL.