rgw/rgw_user.cc: fix possible NULL pointer dereference

CID 1019559 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "usr" to function
  "RGWUser::get_store()", which dereferences it.

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