mds/Migrator.cc: fix dereference after null check

Add asserts to check for 'dir' to fix:

CID 716924 (#1-5 of 5): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "dir" to function "operator
  <<(std::ostream &, CDir &)", which dereferences it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
mds/Server.cc: fix dereference after null check

Add assert to fix:

CID 716925 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "straydn" to function
  "CDentry::get_dir() const", which dereferences it.

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