Disallow setting the AudioServer's 'global_rate_scale' to a value equal or inferior to 0

(cherry picked from commit d7f2c31a05458f672f8f85c4cae6eccb9de09981)

servers/audio_server.cpp

@@ -947,6 +947,7 @@ bool AudioServer::is_bus_channel_active(int p_bus, int p_channel) const {
 }
 
 void AudioServer::set_global_rate_scale(float p_scale) {
+	ERR_FAIL_COND(p_scale <= 0);
 
 	global_rate_scale = p_scale;
 }