Some later code relies on _Layer to set first/last_layer. Make sure it's
always initialized.
Detected by valgrind's conditional jump/move with uninit value logic.
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu> Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>