The gcc undefined behavior sanitizer caught this; surely
any sane memcpy implementation will no-op if size == 0,
but behavior with a *src of NULL is technically undefined
(declared nonnull), so avoid it here.
We are actually in this situation frequently via
xlog_commit_record(), because:
Reported-by: Eric Sandeen <sandeen@redhat.com> Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Eric Sandeen <sandeen@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>