Fix imprecise AD.meet with NULL (closes #566)

Author: sim642

src/cdomains/addressDomain.ml

@@ -125,7 +125,7 @@ struct
       else remove Addr.NullPtr x
     in
     match is_top x, is_top y with
-    | true, true -> uop x y
+    | true, true -> no_null (no_null (uop x y) x) y
     | false, true -> no_null x y
     | true, false -> no_null y x
     | false, false -> cop x y

tests/regression/27-inv_invariants/15-unknown-null-ptr.c

@@ -6,10 +6,10 @@ int main() {
   if (r == NULL)
     assert(r == NULL);
   else
-    assert(r != NULL); // TODO
+    assert(r != NULL);
 
   if (r != NULL)
-    assert(r != NULL); // TODO
+    assert(r != NULL);
   else
     assert(r == NULL);