usb-host: allow emulated (non-async) control requests without USBPacket

xhci needs this for USB_REQ_SET_ADDRESS due to the way
usb addressing is handled by the xhci hardware.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/host-linux.c

diff --git a/hw/usb/host-linux.c b/hw/usb/host-linux.c
index 8df9207..44f1a64 100644
--- a/hw/usb/host-linux.c
+++ b/hw/usb/host-linux.c
@@ -1045,7 +1045,6 @@
 
     /* Note request is (bRequestType << 8) | bRequest */
     trace_usb_host_req_control(s->bus_num, s->addr, p, request, value, index);
-    assert(p->result == 0);
 
     switch (request) {
     case DeviceOutRequest | USB_REQ_SET_ADDRESS:
@@ -1074,6 +1073,7 @@
     }
 
     /* The rest are asynchronous */
+    assert(p && p->result == 0);
 
     if (length > sizeof(dev->data_buf)) {
         fprintf(stderr, "husb: ctrl buffer too small (%d > %zu)\n",