Bluetooth: mesh: Fix uninitialized variable error

This fixes the error below:

main.c: In function ‘bt_mesh_provision’:
main.c:173:17: error: ‘node’ may be used uninitialized in
	       this function [-Werror=maybe-uninitialized]
  173 |                 bt_mesh_cdb_node_del(node, true);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>