memcpy on overlapping memory

[Mark Esler]

Hello,

I believe Gnulib contains memcpy calls on overlapping memory and that
commit 3e7178b337e3a83df9e4f36a4aef516f089e3796 was incomplete.

Please see https://gitlab.com/gsasl/gsasl/-/issues/4 for details.

Best regards,
Mark Esler