C++: Only recognize signed integers as sinks in 'cpp/uncontrolled-arithmetic' in the case of overflow.

cpp/ql/src/Security/CWE/CWE-190/ArithmeticUncontrolled.ql

@@ -82,8 +82,11 @@ predicate missingGuard(VariableAccess va, string effect) {
     op.getUnspecifiedType().(IntegralType).isUnsigned() and
     not op instanceof MulExpr
     or
-    // overflow
-    missingGuardAgainstOverflow(op, va) and effect = "overflow"
+    // overflow - only report signed integer overflow since unsigned overflow
+    // is well-defined.
+    op.getUnspecifiedType().(IntegralType).isSigned() and
+    missingGuardAgainstOverflow(op, va) and
+    effect = "overflow"
   )
 }