Fix crash in AI code when buggy groundnet declares pushback hold point on runway

Some buggy *.groundnet.xml files (as KSEA currently on TS) define the
pushback hold point for some parking positions as a node on a runway.
In this case, this the pushback hold point for parking
'North_Cargo_Ramp', defined as node 5344 in
Airports/K/S/E/KSEA.groundnet.xml, which is defined twice (second error),
first as:

<node index="5344" lat="N47 27.774559" lon="W122 18.465257" isOnRunway="1" holdPointType="PushBack"  />

and then as:

<node index="5344" lat="N47 27.725747" lon="W122 18.159649" isOnRunway="1" holdPointType="PushBack"  />

(due to code in flightgear/src/Airports/dynamicloader.cxx, it should be
the second one that wins, which is not on a runway but on apron in the
north cargo area)

As a consequence, when this gate is selected for an AI aircraft, the
pushback route has only one node (since the pushback hold point is then
the closest point to itself supposedly on runway!), and the
corresponding FGTaxiRoute instance has an empty 'routes' member
variable, which FGTaxiRoute::next() doesn't handle gracefully
(segfault).

It may be that an additional check/change could be desirable in
FGTaxiRoute::next() in such a case (one node and obviously no route in
the FGTaxiRoute instance), however I'm not sure how Durk wants this case
to be handled, since FGTaxiRoute::next() seems to iterate on nodes.

This fixes the bug reported at:

  https://forum.flightgear.org/viewtopic.php?p=308397#p308397 and
  https://sourceforge.net/p/flightgear/mailman/message/35776552/

Thanks to yanfiz and wkitty42 for the report, and to gooneybird for
inspecting the groundnet file.

src/AIModel/AIFlightPlanCreate.cxx

@@ -292,7 +292,8 @@ bool FGAIFlightPlan::createTakeoffTaxi(FGAIAircraft * ac, bool firstFlight,
     if ( runwayNode )
         taxiRoute = gn->findShortestRoute(node, runwayNode);
 
-    if (taxiRoute.empty()) {
+    // This may happen with buggy ground networks
+    if (taxiRoute.size() <= 1) {
         createDefaultTakeoffTaxi(ac, apt, rwy);
         return true;
     }