Various fixes/improvements in connection with Smob allocation

Various fixes/improvements in connection with Smob allocation

These are mostly work towards keeping Guilev2 from crashing all the
time.  Consists of commits:


smobs.tcc: robustify {mark,print}_trampoline in connection with free_smob

Guilev2 has little qualms calling mark/print even after freeing a smob.
So we clean out the pointer to smob data when freeing/unregistering it,
and let the mark/print trampolines detect this condition and punt.


Remove Music_function::mark_smob with ASSERT_LIVE_IS_ALLOWED

GUILEv2's garbage collection is too asynchronous to make this work.


Fix mark_smob definition of (unused) class Smob3