Fix race in io_output() that can lead to double-free

Fix race in io_output() that can lead to double-free

Move call of ioq_makeiov() to its use, because calling it before
empth_select() is racy, as follows.

Player thread flushes output by calling io_output(player->iop, 1).
io_output() sets up iov[] to point to queued output.  empth_select()
blocks on output.

Another thread sends a C_FLASH or C_INFORM message to this player.
This calls io_output(p->iop, 0).  The output file descriptor has
become writable since the player thread blocked on it, so some output
gets written and dequeued.

The player thread resumes, writes out iov[] and dequeues.  Any output
already written by the other thread gets duplicated.  If the other
thread's dequeue operation freed struct io buffers, there's use after
free followed by double-free.