Fix crash in tiny_bvh_speedtest.

[eloj]

Overflow turns index negative. Enforce unsigned result.

[eloj]

[...]
- CPU, coherent, basic 2-way layout, MT, packets (SSE):  2291.45ms for 7.68M rays (3.35MRays/s)
Segmentation fault (core dumped)

$ gdb -q ./tiny_bvh_speedtest core.124460 
Printing of object's derived type based on vtable info is on.
Reading symbols from ./tiny_bvh_speedtest...
[New LWP 124460]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `./tiny_bvh_speedtest'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  main () at tiny_bvh_speedtest.cpp:337
337			rays[i] = rays[j];
(gdb) print i
$1 = 2
(gdb) print j
$2 = -1433861
(gdb) quit