Fix issue #380: Signed integer overflow check

Instead of checking something like `x * y + z > max` where `x * y` can
overflow, check for `x > (max - z) / y` instead.

src/json.hpp.re2c

@@ -9769,19 +9769,19 @@ class basic_json
                 // skip if definitely not an integer
                 if (type != value_t::number_float)
                 {
-                    // multiply last value by ten and add the new digit
+                    auto digit = *curptr - '0';
-                    auto temp = value * 10 + *curptr - '0';
 
-                    // test for overflow
+                    // overflow if value * 10 + digit > max, move terms around
-                    if (temp < value || temp > max)
+                    // to avoid overflow in intermediate values
+                    if (value > (max - digit) / 10)
                     {
                         // overflow
                         type = value_t::number_float;
                     }
                     else
                     {
-                        // no overflow - save it
+                        // no overflow
-                        value = temp;
+                        value = value * 10 + digit;
                     }
                 }
             }