Sonarqube: Make sure "den" can't be zero before doing this division.

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/MathModuleBuiltins.java

@@ -2230,6 +2230,7 @@ static double lanczos_sum(double x) {
                     den = den / x + LANCZOS_DEN_COEFFS[i];
                 }
             }
+            assert den > 0.0 : "den cannot be zero, because LANCZOS_DEN_COEFFS are added";
             return num / den;
         }