examples: count the params as we add them

Count the params as we add them to the param arrays and use that to
update the stream params instead of using hardcoded indexes and sizes.

This makes it easier to add params and it also revealed a miscounted
param.

src/examples/midi-src.c

@@ -178,6 +178,7 @@ int main(int argc, char *argv[])
 	uint8_t buffer[1024];
 	struct spa_pod_builder builder;
 	struct spa_pod *params[1];
+	uint32_t n_params = 0;
 
 	pw_init(&argc, &argv);
 
@@ -226,7 +227,7 @@ int main(int argc, char *argv[])
 	 */
 	spa_pod_builder_init(&builder, buffer, sizeof(buffer));
 
-	params[0] = spa_pod_builder_add_object(&builder,
+	params[n_params++] = spa_pod_builder_add_object(&builder,
 			/* POD Object for the buffer parameter */
 			SPA_TYPE_OBJECT_ParamBuffers, SPA_PARAM_Buffers,
 			/* Default 1 buffer, minimum of 1, max of 32 buffers.
@@ -242,7 +243,7 @@ int main(int argc, char *argv[])
 			SPA_PARAM_BUFFERS_stride,  SPA_POD_Int(1));
 
 	pw_filter_update_params(data.filter, data.port,
-			(const struct spa_pod **)params, SPA_N_ELEMENTS(params));
+			(const struct spa_pod **)params, n_params);
 
 	/* Now connect this filter. We ask that our process function is
 	 * called in a realtime thread. */