mds/Locker.cc: fix explicit null dereferenced

CID 716916 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
  var_deref_model: Passing null pointer "in" to function
  "CInode::is_head()", which dereferences it.

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