Do not return blocked nodes as pathfinder goals

Blocked nodes get skipped by the pathfinding algorithm, resulting in failure. GetNearestNode() now returns the nearest node that is not blocked.

Partially fixes Issue #652

src/ai/PathFinder.cpp

@@ -383,7 +383,7 @@ PathFinder::NodeId PathFinder::getNearestNode(const Vec3f & pos) const {
 
 	for(size_t i = 0; i < map_s; i++) {
 		float dist = arx::distance2(map_d[i].pos, pos);
-		if(dist < distance && map_d[i].nblinked) {
+		if(dist < distance && map_d[i].nblinked && !(map_d[i].flags & ANCHOR_FLAG_BLOCKED)) {
 			best = i;
 			distance = dist;
 		}