mksobj failure

mksobj failure

If mksobj() was told to initialize the object it's creating and the
object class was something it didn't understand, it would issue a
warning and return Null.  But an unknown object class is a severe
internal error and very few callers were prepared to deal with a
Null result, so change mksobj() to panic instead.  Also eliminate the
few attempts to deal with Null result that are present in mkobj.c;
I didn't go looking elsewhere.