hmp: Unbreak "change vnc"

HMP command "change vnc" can take the password as argument, or prompt
for it:

    (qemu) change vnc password 123
    (qemu) change vnc password
    Password: ***
    (qemu)

This regressed in commit cfb5387 "hmp: remove "change vnc TARGET"
command", v6.0.0.

    (qemu) change vnc passwd 123
    Password: ***
    (qemu) change vnc passwd
    (qemu)

The latter passes NULL to qmp_change_vnc_password(), which is a no-no.
Looks like it puts the display into "password required, but none set"
state.

The logic error is easy to miss in review, but testing should've
caught it.

Fix the obvious way.

Fixes: cfb5387
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20210909081219.308065-2-armbru@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

monitor/hmp-cmds.c

@@ -1496,7 +1496,7 @@ void hmp_change(Monitor *mon, const QDict *qdict)
         }
         if (strcmp(target, "passwd") == 0 ||
             strcmp(target, "password") == 0) {
-            if (arg) {
+            if (!arg) {
                 MonitorHMP *hmp_mon = container_of(mon, MonitorHMP, common);
                 monitor_read_password(hmp_mon, hmp_change_read_arg, NULL);
                 return;