Need to NULL check for waylandBufferHandle of m_backBuffer
Need to NULL check for waylandBufferHandle of m_backBuffer

After unmapped, waylandBufferHandle of m_backBuffer can be null
when another buffer attached before sendFrameCallback() called.

Change-Id: I3470e42ca09466c5b2ecc6d6438090f5af50f59d
Reviewed-by: Jørgen Lind <6105bb7d7ebdd613cef5835c4a38d29949bd95ac@digia.com>
Reviewed-by: Andy Nichols <3d1e63b963f99f97839545621c6a9a8e2b2ab8fd@digia.com>

ntanibata/qtwayland,locusf/qtwayland-1,ntanibata/qtwayland,locusf/qtwayland-1,locusf/qtwayland-1,ntanibata/qtwayland,Tofee/qtwayland,Tofee/qtwayland,locusf/qtwayland-1,Tofee/qtwayland