Added a free space function to the T.31 modem in spandsp, so the app doesn't

need to probe around inside the modem.

src/mod/applications/mod_spandsp/mod_spandsp_modem.c

@@ -1279,7 +1279,7 @@ static void *SWITCH_THREAD_FUNC modem_thread(switch_thread_t *thread, void *obj)
 				modem->last_event = switch_time_now();
 			}
 
-			avail = sizeof(buf) - modem->t31_state->tx.in_bytes + modem->t31_state->tx.out_bytes - 1;
+			avail = t31_at_rx_free_space(modem->t31_state);
 
 			if (avail == 0) {
 				switch_log_printf(SWITCH_CHANNEL_LOG, SWITCH_LOG_WARNING, "Buffer Full, retrying....\n");