colorclass: allocate memory to fit the original size of data type

Summary:
The size of Colorclass is 20 bytes, but that of Elm_Color_Overlay 16 bytes.
Currently, there is no code to access last 4 bytes, but it can cause
seg fault by another patch.

Reviewers: cedric, zmike

Differential Revision: https://phab.enlightenment.org/D3784

legacy/elementary/src/lib/elm_color_class.c

@@ -212,7 +212,7 @@ _colorclass_activate(void *data, const Eo_Event *event)
      }
    else
      {
-        cc->current = malloc(sizeof(Elm_Color_Overlay));
+        cc->current = calloc(1, sizeof(Colorclass)); //actually Elm_Color_Overlay
         memcpy(cc->current, ecc, sizeof(Elm_Color_Overlay));
         cc->current->name = eina_stringshare_ref(ecc->name);
      }