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_encoder.c

@@ -41,11 +41,7 @@ encoder_to_stdout(struct encoder *encoder)
 int main(int argc, char **argv)
 {
 	GError *error = NULL;
-	struct audio_format audio_format = {
-		.sample_rate = 44100,
-		.bits = 16,
-		.channels = 2,
-	};
+	struct audio_format audio_format;
 	bool ret;
 	const char *encoder_name;
 	const struct encoder_plugin *plugin;
@@ -66,6 +62,8 @@ int main(int argc, char **argv)
 	else
 		encoder_name = "vorbis";
 
+	audio_format_init(&audio_format, 44100, 16, 2);
+
 	/* create the encoder */
 
 	plugin = encoder_plugin_get(encoder_name);