Fix issue with NaNs

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/VolumeRendering.hlsl

 
     real x = 1 - exp(-extinction * intervalLength);
 
-    // Avoid division by 0.
-    real rcpExt = (extinction != 0) ? rcp(extinction) : 0;
-
-    weight = x * rcpExt;
-    offset = -log(1 - rndVal * x) * rcpExt;
+    weight = x * rcp(extinction);
+    offset = -log(1 - rndVal * x) * rcp(extinction);
 }
 
 // Implements equiangular light sampling.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/Resources/VolumetricLighting.compute

         // We consider it to be constant along the interval [t0, t1] (within the voxel).
         // TODO: piecewise linear.
         float3 scattering = _GlobalFog_Scattering;
-        float  extinction = _GlobalFog_Extinction;
+        float  extinction = max(_GlobalFog_Extinction, FLT_MIN); // Avoid NaNs
         float  asymmetry  = _GlobalFog_Asymmetry;
         
         // TODO: define a function ComputeGlobalFogCoefficients(float3 centerWS),