mds/Server.cc: fix dereference after null check

CID 716926 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing "mdr" to function
  "Server::apply_allocated_inos(MDRequest *)", which dereferences
  null "mdr->session".

Add assert for 'mdr' and assert for session in apply_allocated_inos().

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