[krbdev.mit.edu #6617] uninitialized values used in mkey-migration code

[Ken Raeburn via RT]

kdb5_mkey.c:kdb5_add_mkey declares and uses new_mkey_kvno without  
setting it

No automated tests currently exercise that code anyways, and the tests  
in src/tests/mk_migr don't bother to examine the mkvno values.  (Not  
that you're guaranteed to get something recognizable as garbage, of  
course.)

Ken