qga: fix assert regression on guest-shutdown

Since commit 781f2b3 ("qga: process_event() simplification"),
send_response() is called unconditionally, but will assert when "rsp" is
NULL. This may happen with QCO_NO_SUCCESS_RESP commands, such as
"guest-shutdown".

Fixes: 781f2b3
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Reported-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/main.c

@@ -531,7 +531,11 @@ static int send_response(GAState *s, const QDict *rsp)
     QString *payload_qstr, *response_qstr;
     GIOStatus status;
 
-    g_assert(rsp && s->channel);
+    g_assert(s->channel);
+
+    if (!rsp) {
+        return 0;
+    }
 
     payload_qstr = qobject_to_json(QOBJECT(rsp));
     if (!payload_qstr) {