0.60 hangs at pthread_mutex_lock

Both get_cache_data methods in cache_t.hpp call the mutex lock
a second time without unlocking. The first lock is on entry and
the second from copy. This happens when the lock of n is the
same as that of cache. The mutex will not unlock as the thread
locked it itself, hence the thread thread blocks on the second
lock.


This situation is triggered when calling from pan via gtkspell
although the problem will not be restricted to that combination.