Disable support for constant(zero) as NULL

All front-ends and any other code should take care of generating
constant(NULL) instead.

Author: tautschnig

src/util/expr_util.cpp

@@ -320,5 +320,14 @@ bool is_null_pointer(const constant_exprt &expr)
   if(expr.get_value() == ID_NULL)
     return true;
 
+    // We used to support "0" (when NULL_is_zero), but really front-ends should
+    // resolve this and generate ID_NULL instead.
+#if 0
   return config.ansi_c.NULL_is_zero && expr.value_is_zero_string();
+#else
+  INVARIANT(
+    !expr.value_is_zero_string() || !config.ansi_c.NULL_is_zero,
+    "front-end should use ID_NULL");
+  return false;
+#endif
 }