Mms: Add a check on the Media model

In case there is no image and no video in the slideshow, the
media model might be null.
Add a check to avoid a null pointer access.

Change-Id: I55d63b226835acfa53199eee0d706b8c5ae553c2
Author: Hongyu Zhang <hongyu.zhang@intel.com>
Signed-off-by: Hongyu Zhang <hongyu.zhang@intel.com>
Signed-off-by: Frederic Predon <frederic.predon@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 28037