g10: Fix double free when locating by mbox

* g10/getkey.c (get_best_pubkey_byname): Set new.uid always
to NULL after use.

pubkey_cmp is not guranteed to set new.uid.
So if the diff < 0 case is reached best is set to new.

If then diff > 0 is reached without modifying new.uid
e.g. if the key has no matching mboxes. new.uid is
free'd even though the uid is still referenced in
best.

• GnuPG-Bug-Id: T4462