upgrade_pg.sh: fix check for existing SSL certs

Full file path is needed for the file existence check. The bug was
caused by commit 0d2e6e50

roles/postgres/files/upgrade_pg.sh

@@ -38,9 +38,12 @@ su - postgres -c 'chattr -f +C /var/lib/postgres/data' || :
 su - postgres -c 'initdb --locale en_US.UTF-8 -E UTF8 -D /var/lib/postgres/data'
 vimdiff /var/lib/postgres/{data,data-$FROM_VERSION}/pg_hba.conf
 vimdiff /var/lib/postgres/{data,data-$FROM_VERSION}/postgresql.conf
+
+# copy existing SSL certs from data-$FROM_VERSION to data
 for f in {fullchain,chain,privkey}.pem; do
-	[[ -e $f ]] || continue
-	cp -avx /var/lib/postgres/data-$FROM_VERSION/$f /var/lib/postgres/data/$f
+	if [[ -e /var/lib/postgres/data-$FROM_VERSION/$f ]]; then
+		cp -avx /var/lib/postgres/{data-$FROM_VERSION,data}/$f
+	fi
 done
 
 systemctl stop postgresql.service