MDCache::handle_discover: fix null pointer deref

CID 716990 (#1 of 1): Dereference null return value (NULL_RETURNS)
 dereference: Dereferencing a pointer that might be null "cur" when calling
 "MDCache::replicate_inode(CInode *, int, ceph::bufferlist &)"

Add assert to check for return value from get_inode() as done in other places.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>