tools: bpftool: fix completion for "bpftool map update"

When trying to complete "bpftool map update" commands, the call to
printf would print an error message that would show on the command line
if no map is found to complete the command line.

Fix it by making sure we have map ids to complete the line with, before
we try to print something.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>