Fix two minor bugs (MIGRATE key args and getKeysUsingCommandTable) (#9455)

1. MIGRATE has a potnetial key arg in argv[3]. It should be reflected in the command table.
2. getKeysUsingCommandTable should never free getKeysResult, it is always freed by the caller)
   The reason we never encountered this double-free bug is that almost always getKeysResult
   uses the statis buffer and doesn't allocate a new one.