Fix invalid access to cluster 0 in AAS_AreaRouteToGoalArea()

Newer versions of BSPC such as 2.1h included with the Quake 3 GPL source
code create AAS files containing areas in cluster 0 if the area has no
reachabilities.

The AAS files included with Quake 3 and Team Arena do not contain areas
in cluster 0. It's apparent that BSPC would not create them. Instead it
created clusters with no reachability areas.

It seems the intention was to check if the areanum and goalareanum have
reachable areas using AAS_AreaReachability(areanum) everywhere before
calling AAS_AreaRouteToGoalArea(). This prevents adding cluster 0 to
the routing cache and portal cache. However, it is not checked
everywhere and including some places in the Game VM.

Fix AAS_AreaRouteToGoalArea() instead of trying to wack-a-mole with all
the places that call it.

Cluster 0 access reported by Thomas Köppe (github @tkoeppe) as causing
crashes in rare cases.

code/botlib/be_aas_route.c

@@ -1603,7 +1603,7 @@ int AAS_AreaRouteToGoalArea(int areanum, vec3_t origin, int goalareanum, int tra
 		*reachnum = 0;
 		return qtrue;
 	}
-	//
+	//check !AAS_AreaReachability(areanum) with custom developer-only debug message
 	if (areanum <= 0 || areanum >= aasworld.numareas)
 	{
 		if (botDeveloper)
@@ -1620,6 +1620,10 @@ int AAS_AreaRouteToGoalArea(int areanum, vec3_t origin, int goalareanum, int tra
 		} //end if
 		return qfalse;
 	} //end if
+	if (!aasworld.areasettings[areanum].numreachableareas || !aasworld.areasettings[goalareanum].numreachableareas)
+	{
+		return qfalse;
+	} //end if
 	// make sure the routing cache doesn't grow to large
 	while(AvailableMemory() < 1 * 1024 * 1024) {
 		if (!AAS_FreeOldestCache()) break;