C++: Add another 'good' example for cpp/unsigned-difference-expression-compared-zero.

cpp/ql/src/Security/CWE/CWE-191/UnsignedDifferenceExpressionComparedZero.c

@@ -1,10 +1,14 @@
-unsigned limit = get_limit();
-unsigned total = 0;
+uint32_t limit = get_limit();
+uint32_t total = 0;
 
 while (limit - total > 0) { // BAD: if `total` is greater than `limit` this will underflow and continue executing the loop.
   total += get_data();
 }
 
-while (total < limit) { // GOOD: never underflows.
+while (total < limit) { // GOOD: never underflows here because there is no arithmetic.
+  total += get_data();
+}
+
+while ((int64_t)limit - total > 0) { // GOOD: never underflows here because the result always fits in an `int64_t`.
   total += get_data();
 }