Fix #1260 (crash on Windows/debug, memory error in sending compressed answer with boost::asio)

I tried to make a patch for https://github.com/Project-OSRM/osrm-backend/issues/1260
(based on mentioned stackoverflow ideas)

Checked with Visual Leak Detector - no memory leaks. (there were many of them before addding buffer delete call).

Feel free to replace this patch with some shared_ptr solution if needed, I am just afraid of it (know only new/delete well enough) :)