mds/Server.cc: fix dereference after null check

Move code block up into already existing check for destdn != NULL.

CID 1019556 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "destdn" to function
  "CDentry::push_projected_linkage()", which dereferences it.

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