readline: fix hmp completion issue

readline: fix hmp completion issue

The auto completion does not work in some cases.

Case 1.

1. (qemu) info reg
2. Press 'Tab'.
3. It does not auto complete.

Case 2.

1. (qemu) block_resize flo
2. Press 'Tab'.
3. It does not auto complete 'floppy0'.

Since the readline_add_completion_of() may add any completion when
strlen(pfx) is zero, we remove the check with (name[0] == '\0') because
strlen() always returns zero in that case.

Fixes: 52f50b1e9f8f ("readline: Extract readline_add_completion_of() from monitor")
Cc: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>