APM: fixed overflow in geofence code for altitudes above 327 meters

ArduPlane/geofence.pde

@@ -151,7 +151,7 @@ static bool geofence_check_minalt(void)
     if (g.fence_minalt == 0) {
         return false;
     }
-    return (current_loc.alt < (g.fence_minalt*100) + home.alt);
+    return (current_loc.alt < (g.fence_minalt*100.0) + home.alt);
 }
 
 /*
@@ -165,7 +165,7 @@ static bool geofence_check_maxalt(void)
     if (g.fence_maxalt == 0) {
         return false;
     }
-    return (current_loc.alt > (g.fence_maxalt*100) + home.alt);
+    return (current_loc.alt > (g.fence_maxalt*100.0) + home.alt);
 }
 
 
@@ -263,7 +263,7 @@ static void geofence_check(bool altitude_check_only)
             // invalid min/max, use RTL_altitude
             guided_WP.alt = home.alt + g.RTL_altitude;
         } else {
-            guided_WP.alt = home.alt + 100*(g.fence_minalt + g.fence_maxalt)/2;
+            guided_WP.alt = home.alt + 100.0*(g.fence_minalt + g.fence_maxalt)/2;
         }
         guided_WP.id = 0;
         guided_WP.p1  = 0;