nir: initialize value in copy_prop_vars_block

Fixes following valgrind warning:

   ==27561== Conditional jump or move depends on uninitialised value(s)
   ==27561==    at 0x667856B: value_set_ssa_components (nir_opt_copy_prop_vars.c:78)
   ==27561==    by 0x667A1C4: copy_prop_vars_block (nir_opt_copy_prop_vars.c:797)

Fixes: 62332d13 "nir: Add a local variable-based copy propagation pass"
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 22267fef)