fix bigint overflow bug

Author: 9il

source/mir/bignum/decimal.d

@@ -981,7 +981,8 @@ struct Decimal(size_t maxSize64)
     ref opOpAssign(string op, size_t rhsMaxSize64)(ref const Decimal!rhsMaxSize64 rhs) @safe pure return
         if (op == "+" || op == "-")
     {
-        BigInt!rhsMaxSize64 rhsCopy;
+        import mir.utility: max;
+        BigInt!(max(rhsMaxSize64, maxSize64, 256u)) rhsCopy = void;
         BigIntView!(const size_t) rhsView;
         auto expDiff = exponent - rhs.exponent;
         if (expDiff >= 0)

source/mir/math/sum.d

@@ -38,6 +38,8 @@ unittest
     assert(ar.sum!"kbn" == 0.7000000000000455);
     assert(ar.sum!"kb2" == 0.7000000000000455);
     assert(ar.sum!"precise" == 0.7000000000000455);
+
+    assert([1e-20, 1].sum!"decimal" == 1);
 }
 
 ///