Plane: use 4 degrees if zero takeoff pitch

ArduPlane/commands_logic.cpp

@@ -320,6 +320,10 @@ void Plane::do_takeoff(const AP_Mission::Mission_Command& cmd)
     set_next_WP(cmd.content.location);
     // pitch in deg, airspeed  m/s, throttle %, track WP 1 or 0
     auto_state.takeoff_pitch_cd        = (int16_t)cmd.p1 * 100;
+    if (auto_state.takeoff_pitch_cd <= 0) {
+        // if the mission doesn't specify a pitch use 4 degrees
+        auto_state.takeoff_pitch_cd = 400;
+    }
     auto_state.takeoff_altitude_rel_cm = next_WP_loc.alt - home.alt;
     next_WP_loc.lat = home.lat + 10;
     next_WP_loc.lng = home.lng + 10;