mds/MDCache.cc: fix dereference NULL pointer

Fix possible NULL pointer dereference. Change return value of
CInode::get_dirfrag() to return NULL instead of 0 since it's a
pointer returned.

CID 716991 Dereference null return value (CWE-478)

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