libpng: Fix for memory leak while indexing the PNG images

de-allocating the index resorces at proper place. png_read_destroy()
making png_ptr->index NULL and due to this de-allocation was not
happening even though code exist (it was like dead-code).

Change-Id: I770bbfc486365b1a8b6a8dc5f128aa2b3a34cd50
CRs-Fixed: 551617