db-ctl-base: Fix uninitialized datum fields while checking conditions.

Statically allocated datum objects should be properly initialized with
a special function instead of doing that manually.

 WARNING: MemorySanitizer: use-of-uninitialized-value
  0 0x58789c in ovsdb_datum_compare_3way lib/ovsdb-data.c:1846:19
  1 0x52bbab in evaluate_relop lib/db-ctl-base.c:731:16
  2 0x52b042 in check_condition lib/db-ctl-base.c:844:22
  3 0x522fea in cmd_wait_until lib/db-ctl-base.c:1935:22
  4 0x4c704b in do_vsctl utilities/ovs-vsctl.c:3001:13
  5 0x4c4429 in main utilities/ovs-vsctl.c:204:17
  6 0x7f5ad5 in __libc_start_call_main
  7 0x7f5ad5 in __libc_start_main@GLIBC_2.2.5
  8 0x432b04 in _start (utilities/ovs-vsctl+0x432b04)

In this case the reference counter ended up not initialized.

Fixes: 485ac63 ("ovsdb: Add lazy-copy support for ovsdb_datum objects.")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/db-ctl-base.c

@@ -831,14 +831,10 @@ check_condition(const struct ovsdb_idl_table_class *table,
         } else {
             struct ovsdb_datum a;
 
+            ovsdb_datum_init_empty(&a);
             if (found) {
                 a.n = 1;
                 a.keys = &have_datum->values[idx];
-                a.values = NULL;
-            } else {
-                a.n = 0;
-                a.keys = NULL;
-                a.values = NULL;
             }
 
             retval = evaluate_relop(&a, &b, &type, operator);