MINOR: pass a valid buffer pointer to ha_thread_dump_one()

MINOR: pass a valid buffer pointer to ha_thread_dump_one()

The goal is to let the caller deal with the pointer so that the function
only has to fill that buffer without worrying about locking. This way,
synchronous dumps from "show threads" are produced and emitted directly
without causing undesired locking of the buffer nor risking causing
confusion about thread_dump_buffer containing bits from an interrupted
dump in progress.

It's only the caller that's responsible for notifying the requester of
the end of the dump by setting bit 0 of the pointer if needed (i.e. it's
only done in the debug handler).