Fix for gcc6 -Wnarrowing error.

--- tests/iodelay.cpp	2014-07-18 14:28:13.000000000 +0200
+++ tests/iodelay.cpp	2016-09-11 18:12:00.563013354 +0200
@@ -167,8 +167,8 @@
 static jack_port_t    *jack_capt;
 static jack_port_t    *jack_play;
 
-jack_latency_range_t   capture_latency = {-1, -1};
-jack_latency_range_t   playback_latency = {-1, -1};
+jack_latency_range_t   capture_latency = {(jack_nframes_t)-1, (jack_nframes_t)-1};
+jack_latency_range_t   playback_latency = {(jack_nframes_t)-1, (jack_nframes_t)-1};
 
 void
 latency_cb (jack_latency_callback_mode_t mode, void *arg)
@@ -266,4 +266,4 @@
     return 0;
 }
 
-// --------------------------------------------------------------------------------
\ Kein Zeilenumbruch am Dateiende.
+// --------------------------------------------------------------------------------