Reset pointer to 0 after freeing

set m_textureResource to 0 after freeing it, to
ensure we allocate a new one two lines below before
using it again 5 lines below.

Change-Id: Ib9c4de94aefed91f15d0fab0cd3c774a64f2f891
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>