g10: Fix double free when locating by mbox

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
(cherry picked from commit e57954ed278cb5e6e725005b1ecaf7ce70006ce0)
(cherry picked from commit 35899dc2903b118620e6f9f0fa6b21c8568abbf1)

Gbp-Pq: Topic from-2.2.16
Gbp-Pq: Name g10-Fix-double-free-when-locating-by-mbox.patch