Fix rare NaN case in PowerHeuristic()

If the first PDF was too large, it would become infinity when it was
squared, which in turn would lead to a NaN being returned.

src/pbrt/util/sampling.h

@@ -70,6 +70,8 @@ PBRT_CPU_GPU inline Float BalanceHeuristic(int nf, Float fPdf, int ng, Float gPd
 
 PBRT_CPU_GPU inline Float PowerHeuristic(int nf, Float fPdf, int ng, Float gPdf) {
     Float f = nf * fPdf, g = ng * gPdf;
+    if (IsInf(Sqr(f)))
+        return 1;
     return Sqr(f) / (Sqr(f) + Sqr(g));
 }