BUG/MEDIUM: server: fix potential null-deref after previous fix

BUG/MEDIUM: server: fix potential null-deref after previous fix

A valid build warning was reported in the CI with latest commit b40ce97ecc
("BUG/MEDIUM: server: fix crash after duplicate GUID insertion"). Indeed,
if the first test in the function fails, we branch to the err label
with guid==NULL and will crash there. Let's just test guid before
dereferencing it for freeing.

This needs to be backported to 3.0 as well since the commit above was
meant to go there.