main: Prevent capture restart if idle

This prevents restarting the capture on dimension change when idle.
Previously the capture was running after the dimensions had changed
which caused starting the capture once a client connects to fail.

Fixes #152

src/main.c

@@ -614,6 +614,9 @@ void on_output_dimension_change(struct output* output)
 	struct wayvnc* self = output->userdata;
 	assert(self->selected_output == output);
 
+	if (self->nr_clients == 0)
+		return;
+
 	nvnc_log(NVNC_LOG_DEBUG, "Output dimensions changed. Restarting frame capturer...");
 
 	screencopy_stop(&self->screencopy);