Fix rare bug in BSSRDF surface intersection code
Fix rare bug in BSSRDF surface intersection code

Due to round-off error, sometimes we'd try to shoot a ray to the offset
point we were starting from, leading to a (0,0,0)-valued direction.  Stop
looking for more intersections when this happens.

mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3