[FIX] Check function pointers in dllucal_exit()

If there is an error on startup before dllucal_init() is called,
the function pointers will not be set. Therefore a call to
dllucal_exit() causes a crash. Now, the function pointers will
be checked before calling them.

Change-Id: Ic1ecf66e1117101b053924a031a5d1e24f13c5cd