Plane: fixed an integer multiply error that caused poor altitude on landing

the control of altitude between waypoints was broken due to an integer
overflow

ArduPlane/commands.pde

@@ -184,10 +184,13 @@ static void set_next_WP(struct Location *wp)
     // -----------------------------------------------
     target_altitude_cm = current_loc.alt;
 
-    if(prev_WP.id != MAV_CMD_NAV_TAKEOFF && prev_WP.alt != home.alt && (next_WP.id == MAV_CMD_NAV_WAYPOINT || next_WP.id == MAV_CMD_NAV_LAND))
+    if (prev_WP.id != MAV_CMD_NAV_TAKEOFF && 
+        prev_WP.alt != home.alt && 
+        (next_WP.id == MAV_CMD_NAV_WAYPOINT || next_WP.id == MAV_CMD_NAV_LAND)) {
         offset_altitude_cm = next_WP.alt - prev_WP.alt;
-    else
-        offset_altitude_cm = 0;
+    } else {
+        offset_altitude_cm = 0;        
+    }
 
     // zero out our loiter vals to watch for missed waypoints
     loiter_delta            = 0;

ArduPlane/navigation.pde

@@ -113,9 +113,9 @@ static void calc_bearing_error()
 
 static void calc_altitude_error()
 {
-    if(control_mode == AUTO && offset_altitude_cm != 0) {
+    if (control_mode == AUTO && offset_altitude_cm != 0) {
         // limit climb rates
-        target_altitude_cm = next_WP.alt - ((float)((wp_distance -30) * offset_altitude_cm) / (float)(wp_totalDistance - 30));
+        target_altitude_cm = next_WP.alt - (offset_altitude_cm*((float)(wp_distance-30) / (float)(wp_totalDistance-30)));
 
         // stay within a certain range
         if(prev_WP.alt > next_WP.alt) {