Fix for data race in plans_finish_rd

[ceeac]

The read access to min_height and max_height at the beginning of karte_t::plans_finish_rd needs to be synchronized since another thread might be writing to any of these two variables.

[Dwachs]

Imho no mutex is needed. It would be sufficient to initialize those values with -128 and +127. The writing to min//max_height is already protected by some mutex.

[ceeac]

Yes, that would work too.

[Dwachs]

implemented in r9060