zsh: ensure we have enough commands to store in the cache

Otherwise, the cache would be invalid and won't be refreshed soon. This
can happen when the user has the completion installed before docker is
installed.

Signed-off-by: Vincent Bernat <vincent@bernat.im>

contrib/completion/zsh/_docker

@@ -431,7 +431,7 @@ __docker_commands() {
         lines=(${(f)"$(_call_program commands docker 2>&1)"})
         _docker_subcommands=(${${${lines[$((${lines[(i)Commands:]} + 1)),${lines[(I)    *]}]}## #}/ ##/:})
         _docker_subcommands=($_docker_subcommands 'daemon:Enable daemon mode' 'help:Show help for a command')
-        _store_cache docker_subcommands _docker_subcommands
+        (( $#_docker_subcommands > 2 )) && _store_cache docker_subcommands _docker_subcommands
     fi
     _describe -t docker-commands "docker command" _docker_subcommands
 }