Handle zero case

packages/contracts/src/2.0.0/protocol/Exchange/libs/LibMath.sol

@@ -60,14 +60,26 @@ contract LibMath is
         pure
         returns (bool isError)
     {
+        require(denominator > 0, "DIVISION_BY_ZERO");
+        
         // The absolute rounding error is the difference between the rounded
         // value and the ideal value. The relative rounding error is the
         // absolute rounding error divided by the absolute value of the
-        // ideal value. We want the relative rounding error to be strictly less
+        // ideal value. This is undefined when the ideal value is zero.
-        // than 0.1%.
+        //
-        // Let's call `numerator * target % denominator` the remainder.
         // The ideal value is `numerator * target / denominator`.
+        // Let's call `numerator * target % denominator` the remainder.
         // The absolute error is `remainder / denominator`.
+        //
+        // When the ideal value is zero, we require the absolute error to
+        // be zero. Fortunately, this is always the case. The ideal value is
+        // zero iff `numerator == 0` and/or `target == 0`. In this case the
+        // remainder and absolute error are also zero. 
+        if (target == 0 || numerator == 0) {
+            return false;
+        }
+        // Otherwise, we want the relative rounding error to be strictly
+        // less than 0.1%.
         // The relative error is `remainder / numerator * target`.
         // We want the relative error less than 1 / 1000:
         //        remainder / numerator * denominator  <  1 / 1000