evas: If there isn't clipper when recalcing clip, set mask.clip = NULL.

evas: If there isn't clipper when recalcing clip, set mask.clip = NULL.

Summary:
There's problem in Tizen3.0.

1. Clip set mask_obj to obj for masking.
2. Unset mask_obj from obj, and del mask_obj.
3. obj has clip.mask still. So obj is trying to do mask_subrender() for freeed mask_obj.

So reset clip.mask to NULL, If there isn't clipper.

Now, there's no routine for reseting clip.mask when clipper object is freed. isn't it?
Actually I'm not sure that clip.mask=NULL should be there as this patch.

Test Plan: Tizen3.0 wearable

Reviewers: cedric, raster, wonsik, jpeg

Subscribers: scholb.kim, dkdk

Differential Revision: https://phab.enlightenment.org/D4721

Signed-off-by: Jean-Philippe Andre <jp.andre@samsung.com>