Early return in case if bandwidth limiting disabled

In case when a lot of peers used (in my case 1000+) - profiler showed cpu usage (in code i see at least 2 full loops for full list of peers) inside enet_host_bandwidth_throttle function - even in case when bandwidth limiting disabled.

PS: Sorry i am new with git - so only "early exit in case of no bandwith throttle" change need to be used.