lib/a_star: fix use of virtual type in Node::path_to

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/a_star: minor style cleanup

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge remote-tracking branch 'alexis/a_star'

The type error was detected by nitg in complex cases.