cli: remove one more g_assert()

I pushed accidentally pushed commit 9702310f25 ('clients: bulk removal
of g_assert*() statements') earlier than I intended, without addressing
one more case introduced by preceding merge. Fix it now.

Fixes: 9702310f25 ('clients: bulk removal of g_assert*() statements')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1166

src/nmcli/connections.c

@@ -9723,7 +9723,7 @@ do_connection_migrate(const NMCCommand *cmd, NmCli *nmc, int argc, const char *c
             gs_free char *line = NULL;
 
             /* nmc_do_cmd() should not call this with argc=0. */
-            g_assert(!nmc->complete);
+            g_return_if_fail(!nmc->complete);
 
             line = nmc_readline(&nmc->nmc_config, PROMPT_CONNECTIONS);
             nmc_string_to_arg_array(line, NULL, TRUE, &arg_arr, &arg_num);