Fix leak in DB2 krb5_db_promote() implementation

In krb5_db2_merge_nra_iterator(), free the entry loaded from the
source DB.

(cherry picked from commit 0c5f74a)

ticket: 8442
version_fixed: 1.13.7

src/plugins/kdb/db2/kdb_db2.c

@@ -1399,6 +1399,7 @@ krb5_db2_merge_nra_iterator(krb5_pointer ptr, krb5_db_entry *entry)
         retval = 0;
     }
 
+    krb5_db_free_principal(nra->kcontext, s_entry);
     return retval;
 }