Tighten behavior of _rs_allocate failure for portable arc4random implementations.

Tighten behavior of _rs_allocate failure for portable arc4random implementations.

In the event of a failure in _rs_allocate for rsx, we still have a reference to
freed memory for rs on return. Not a huge deal since we subsequently abort in
_rs_init, but it looks strange on its own.

ok deraadt@