Fix indirect loading of Eigen::Ref

Put the caster's temporary array on life support to ensure correct
lifetime when it's being used as a subcaster.
diff --git a/include/pybind11/eigen.h b/include/pybind11/eigen.h
index 53d4dab..fc07051 100644
--- a/include/pybind11/eigen.h
+++ b/include/pybind11/eigen.h
@@ -462,6 +462,7 @@
             if (!fits || !fits.template stride_compatible<props>())
                 return false;
             copy_or_ref = std::move(copy);
+            loader_life_support::add_patient(copy_or_ref);
         }
 
         ref.reset();