(trunk libT) #4209 "Shortcut UDP tracker test" -- the goal of #4209 is to minimize...

(trunk libT) #4209 "Shortcut UDP tracker test" -- the goal of #4209 is to minimize the cost of the UDP event callback function, so apply the heap pruning eye to that by removing an unnecessary malloc/free call there.

Previously we allocated a 4096 character buffer each time; now we allocate it on the stack. It seems all the distros and OS flavors that Transmission runs on have multi-MB default stack sizes, so a hardwired 4K array should be safe.