Fix bug uncovered with ground approximation code.

lib/boot.ks

@@ -37,7 +37,7 @@ function Bootstrap {
     } else {
       set CORE:BOOTFILENAME to "/init".
     }
-//    reboot.
+    reboot.
   }
 }
 

lib/navigation.ks

@@ -33,6 +33,9 @@ function CreateCircularizationNode {
 // REQUIRES a circular orbit.
 // TODO: it would be great to semi-automate this...
 function TargetBurnDistance {
+  if not HASTARGET {
+    return -1.
+  }
   return Sqrt(StoppingDistance()^2 + (SHIP:ORBIT:SEMIMAJORAXIS - SHIP:BODY:RADIUS - TARGET:ALTITUDE)^2).
 }