Fix addDownloadResult infinite loop when max-download-results is reached (take 2)

RequestGroupMan::addDownloadResult will to purge the oldest result according to max-download-results, or at least this is how it is supposed to be.

The implementation however does not actually purge anything, ever. The while loop is therefore an infinite busy loop, halting aria2 while buring CPU circles.

Forgive my somewhat rusty C++/STL

The first take one was actually quite wrong :p