Fix 'return foo()' in order to use Clang

We need to compile with Clang, since the GCC version used by Android NDK r9d does not compile some C++11 features.

Clang complains about 'return arc4random_buf()' in a function that should return void. So, now it first calls arc4random_buf() and then returns. Similar patches have been commited on LibEvent master, but not backported to the branch patches-2.0. Since automatic cherry-pick from the master creates conflicts, we applied them by hand.