vulkan/overlay-layer: fix cast errors

Not quite sure what version of GCC/Clang produces errors (8.3.0
locally was fine).

v2: also fix an integer literal issue (Karol)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 2d292793)