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: 52f50b1e ("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>