Copter: bug fix for take-off so it never descends

If copter was already flying when take-off command was received it could
descend to the target altitude if it was below the current altitude.

ArduCopter/commands_logic.pde

@@ -245,7 +245,7 @@ static void do_takeoff()
 
     // Set wp navigation target to safe altitude above current position
     Vector3f pos = inertial_nav.get_position();
-    pos.z = command_nav_queue.alt;
+    pos.z = max(pos.z, command_nav_queue.alt);
     wp_nav.set_destination(pos);
 
     // prevent flips