Fixed memory leak when insertion fails.

Hey Jeff, I ended up testing the quadtree with valgrind and encountered a little bit of a leak. I seem to have fixed the problem with these changes. Also, I had to add the "-lm" flag to compile on my system. Anyway, thought I should let you know what I encountered. Feel free to test for yourself.

$ valgrind -q --leak-check=full ./bin/benchmark
$ valgrind -q --leak-check=full ./bin/test