minios: clean up unneeded "err = NULL" in frontend drivers

minios: clean up unneeded "err = NULL" in frontend drivers

This patch removes cases where the error message pointer is already
NULL and is then set to NULL. These are harmless, but suggest
incorrect practice: the pointer should be passed to free() to
deallocate memory prior to reassignment. There are no functional
changes in this patch.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
Reviewed-by: Matt Wilson <msw@amazon.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
[msw: split a larger patch from Ben into this cleanup patch]
Signed-off-by: Matt Wilson <msw@amazon.com>