adding lock to planner makePlan fail case

A planner mutex is created in the [planner thread](https://github.com/phil0stine/navigation/blob/35eba4f3d02efd6a5c14c2ac4244bdb6dd3b9027/move_base/src/move_base.cpp#L598-619), then locked and unlocked.

`makePlan` is called _after_ the mutex is unlocked, and in the meantime, it is possible the controller has executed its recovery behaviors, set `runPlanner_` to [false](https://github.com/phil0stine/navigation/blob/35eba4f3d02efd6a5c14c2ac4244bdb6dd3b9027/move_base/src/move_base.cpp#L987-989), and called [resetState](https://github.com/phil0stine/navigation/blob/35eba4f3d02efd6a5c14c2ac4244bdb6dd3b9027/move_base/src/move_base.cpp#L1145-1149), which sets `state_` to PLANNING from CLEARING.

Now, when `makePlan` returns, if `gotPlan` is false, it will fall into the second [condition](https://github.com/phil0stine/navigation/blob/35eba4f3d02efd6a5c14c2ac4244bdb6dd3b9027/move_base/src/move_base.cpp#L647), since `state_` has just been set to PLANNING.

Then, it will set `state_` to CLEARING, which will be the current state the next time a goal is sent to move_base. Even if the new goal is valid, a recovery behavior will be performed.