mathutil: Fix scaling BoundingSphere to infinite causing assertions

This error occurs when a BoundingSphere with a large radius is scaled by an even larger radius such that the radius becomes infinite.  In this case, the BoundingSphere should be properly marked as infinite so that it behaves properly (and doesn't cause other assertions down the line).

panda/src/mathutil/boundingSphere.cxx

@@ -112,6 +112,10 @@ xform(const LMatrix4 &mat) {
 
     // Transform the center
     _center = _center * mat;
+
+    if (cinf(_radius)) {
+      set_infinite();
+    }
   }
 }