Add audio_format_init() function

It makes no difference right now, but we're about to add an endianness
flag and will want to make sure it's correctly initialised every time.

test/run_filter.c

@@ -70,11 +70,7 @@ load_filter(const char *name)
 
 int main(int argc, char **argv)
 {
-	struct audio_format audio_format = {
-		.sample_rate = 44100,
-		.bits = 16,
-		.channels = 2,
-	};
+	struct audio_format audio_format;
 	bool success;
 	GError *error = NULL;
 	struct filter *filter;
@@ -87,6 +83,8 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
+	audio_format_init(&audio_format, 44100, 16, 2);
+
 	g_thread_init(NULL);
 
 	/* read configuration file (mpd.conf) */