[Fix] Fixed seg fault when trying to land in a system with no planets.

dat/start.xml

@@ -4,8 +4,8 @@
  <player>
   <ship>TestShip</ship>
   <credits>
-   <low>10000</low>
-   <high>20000</high>
+   <low>1000000000000</low>
+   <high>9999999999999999</high>
   </credits>
   <date>
    <low>1300</low>

src/player.c

@@ -1689,6 +1689,12 @@ void player_land(void) {
     return;
   }
 
+  /* Check if there are planets to land on. */
+  if(cur_system->nplanets == 0) {
+    player_message("There are no planets to land on.");
+    return;
+  }
+
   Planet* planet = cur_system->planets[planet_target];
   if(planet_target >= 0) {
     if(!planet_hasService(planet, PLANET_SERVICE_LAND)) {