Task: Correct a false-positive warning

- The command "task 1 modify due: wait:2days" gave the following
   output:

    Warning: You have specified that the 'wait' date is after the
    'due' date.

  Now, we check if the due date is zero, in which case the due date is
  being removed.

src/Task.cpp

@@ -1572,7 +1572,8 @@ void Task::validate_before (const std::string& left, const std::string& right)
     Date date_left (get_date (left));
     Date date_right (get_date (right));
 
-    if (date_left > date_right)
+    // if date is zero, then it is being removed (e.g. "due: wait:1day")
+    if (date_left > date_right && date_right != 0)
       context.footnote (format (STRING_TASK_VALID_BEFORE, left, right));
   }
 #endif