raft: Fix leak of the incomplete command.

Function raft_command_initiate() returns correctly referenced command
instance.  'n_ref' equals 1 for complete commands and 2 for incomplete
commands because one more reference is in raft->commands list.
raft_handle_execute_command_request__() leaks the reference by not
returning pointer anywhere and not unreferencing incomplete commands.

 792 bytes in 11 blocks are definitely lost in loss record 258 of 262
    at 0x483BB1A: calloc (vg_replace_malloc.c:762)
    by 0x44BA32: xcalloc (util.c:121)
    by 0x422E5F: raft_command_create_incomplete (raft.c:2038)
    by 0x422E5F: raft_command_initiate (raft.c:2061)
    by 0x428651: raft_handle_execute_command_request__ (raft.c:4161)
    by 0x428651: raft_handle_execute_command_request (raft.c:4177)
    by 0x428651: raft_handle_rpc (raft.c:4230)
    by 0x428651: raft_conn_run (raft.c:1445)
    by 0x428DEA: raft_run (raft.c:1803)
    by 0x407392: main_loop (ovsdb-server.c:226)
    by 0x407392: main (ovsdb-server.c:469)

Fixes: 1b1d2e6 ("ovsdb: Introduce experimental support for clustered databases.")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>

ovsdb/raft.c

@@ -3960,9 +3960,7 @@ raft_handle_execute_command_request__(
     cmd->sid = rq->common.sid;
 
     enum raft_command_status status = cmd->status;
-    if (status != RAFT_CMD_INCOMPLETE) {
-        raft_command_unref(cmd);
-    }
+    raft_command_unref(cmd);
     return status;
 }