Map mutexes do nothing

[skohlbr]

The mutexes used to lock the map structure do nothing, as only temporaries are created that go out of scope immediately after creation. Instead of

boost::mutex::scoped_lock(map_mutex_);

this should be used:

boost::mutex::scoped_lock lock (map_mutex_);

I only noticed this bug when carelessly copying some code from slam_gmapping and then getting crashes. Apparently, the mutexes aren´t that crucial in slam_gmapping though, as this bug has been there from the beginning years ago :)

[wjwwood]

⁉️ Oh man, this is silly... I'll make a pull request...