Fix some Cycles random walk SSS precision issues with small radius

Referenced by issue #64620, Random Walk SSS in Principled BSDF gives very dark result for small subsurface inputs

intern/cycles/kernel/kernel_subsurface.h

@@ -605,6 +605,13 @@ ccl_device_noinline
     if (hit) {
       t = ray->t;
     }
+    else if (bounce == 0) {
+      /* Restore original position if nothing was hit after the first bounce.
+       * Otherwise if the ray_offset() to avoid self-intersection is relatively
+       * large compared to the scattering radius, we go never backup high enough
+       * to exit the surface. */
+      ray->P = sd->P;
+    }
 
     /* Advance to new scatter location. */
     ray->P += t * ray->D;