Initialize cluster owner_not_claiming_slot to avoid warning (#12391)

valgrind report a Uninitialised warning:
```
==25508==  Uninitialised value was created by a heap allocation
==25508==    at 0x4848899: malloc (in
/usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==25508==    by 0x1A35A1: ztrymalloc_usable_internal (zmalloc.c:117)
==25508==    by 0x1A368D: zmalloc (zmalloc.c:145)
==25508==    by 0x21FDEA: clusterInit (cluster.c:973)
==25508==    by 0x19DC09: main (server.c:7306)
```

Introduced in #12344

src/cluster.c

@@ -998,6 +998,8 @@ void clusterInit(void) {
     memset(server.cluster->slots,0, sizeof(server.cluster->slots));
     clusterCloseAllSlots();
 
+    memset(server.cluster->owner_not_claiming_slot, 0, sizeof(server.cluster->owner_not_claiming_slot));
+
     /* Lock the cluster config file to make sure every node uses
      * its own nodes.conf. */
     server.cluster_config_file_lock_fd = -1;