Fix for #5041

In Alloc, on arena failure the code assigns memptr = nullptr instead of *memptr = nullptr.

Summary

Additional background

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate