Copter: fixed vertical speed init for ALT_HOLD mode

ArduCopter/control_althold.pde

@@ -9,6 +9,10 @@ static bool althold_init(bool ignore_checks)
 {
     // initialise altitude target to stopping point
     pos_control.set_target_to_stopping_point_z();
+
+    // initialize vertical speeds and leash lengths
+    pos_control.set_speed_z(-g.pilot_velocity_z_max, g.pilot_velocity_z_max);
+
     return true;
 }