CPP: Add support for += and -=.

Author: geoffw0

cpp/ql/src/semmle/code/cpp/security/Overflow.qll

@@ -63,6 +63,7 @@ predicate missingGuardAgainstOverflow(Operation e, VariableAccess use) {
   exists(LocalScopeVariable v | use.getTarget() = v |
     // overflow possible if large
     (e instanceof AddExpr and not guardedLesser(e, varUse(v))) or
+    (e instanceof AssignAddExpr and not guardedLesser(e, varUse(v))) or
     (e instanceof IncrementOperation and not guardedLesser(e, varUse(v)) and v.getType().getUnspecifiedType() instanceof IntegralType) or     
     // overflow possible if large or small
     (e instanceof MulExpr and
@@ -76,6 +77,7 @@ predicate missingGuardAgainstUnderflow(Operation e, VariableAccess use) {
   exists(LocalScopeVariable v | use.getTarget() = v |
     // underflow possible if use is left operand and small
     (use = e.(SubExpr).getLeftOperand() and not guardedGreater(e, varUse(v))) or
+    (use = e.(AssignSubExpr).getLValue() and not guardedGreater(e, varUse(v))) or
     // underflow possible if small
     (e instanceof DecrementOperation and not guardedGreater(e, varUse(v)) and v.getType().getUnspecifiedType() instanceof IntegralType) or
     // underflow possible if large or small