mds/CInode.cc: fix FORWARD_NULL issue

mds/CInode.cc:  fix FORWARD_NULL issue

Don't check for the conditions used inside get_projected_parent_dn(),
simply check if the returned value is valid.

Fix for:

CID 716915: Dereference after null check (FORWARD_NULL)
 var_deref_model: Passing null pointer this->get_projected_parent_dn()
 to pre_dirty, which dereferences it.

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