sndio: use sio_nfds

Instead of guessing the number of needed pollfd structures, use sio_nfds
and allocate the number of required structures.
If libsndio uses its alsa backend then in my tests the number of
required pollfds is 16, larger than the previously defined MAXFDs.
This resulted in sio_pollfd overwriting stack memory.

src/cubeb_sndio.c

@@ -32,6 +32,7 @@
   X(sio_eof)                                 \
   X(sio_getpar)                              \
   X(sio_initpar)                             \
+  X(sio_nfds)                                \
   X(sio_onmove)                              \
   X(sio_open)                                \
   X(sio_pollfd)                              \
@@ -135,18 +136,23 @@ sndio_onmove(void *arg, int delta)
 static void *
 sndio_mainloop(void *arg)
 {
-#define MAXFDS 8
-  struct pollfd pfds[MAXFDS];
+  struct pollfd *pfds;
   cubeb_stream *s = arg;
   int n, eof = 0, prime, nfds, events, revents, state = CUBEB_STATE_STARTED;
   size_t pstart = 0, pend = 0, rstart = 0, rend = 0;
   long nfr;
 
+  nfds = WRAP(sio_nfds)(s->hdl);
+  pfds = calloc(nfds, sizeof (struct pollfd));
+  if (pfds == NULL)
+	  return NULL;
+
   DPR("sndio_mainloop()\n");
   s->state_cb(s, s->arg, CUBEB_STATE_STARTED);
   pthread_mutex_lock(&s->mtx);
   if (!WRAP(sio_start)(s->hdl)) {
     pthread_mutex_unlock(&s->mtx);
+    free(pfds);
     return NULL;
   }
   DPR("sndio_mainloop(), started\n");
@@ -274,6 +280,7 @@ sndio_mainloop(void *arg)
   s->hwpos = s->swpos;
   pthread_mutex_unlock(&s->mtx);
   s->state_cb(s, s->arg, state);
+  free(pfds);
   return NULL;
 }