usb/xhci: avoid trigger assertion if guest write wrong epid

we found the following core in our environment:
0  0x00007fc6b06c2237 in raise ()
1  0x00007fc6b06c3928 in abort ()
2  0x00007fc6b06bb056 in __assert_fail_base ()
3  0x00007fc6b06bb102 in __assert_fail ()
4  0x0000000000702e36 in xhci_kick_ep (...)
5  0x000000000047897a in memory_region_write_accessor (...)
6  0x000000000047767f in access_with_adjusted_size (...)
7  0x000000000047944d in memory_region_dispatch_write (...)
(mr=mr@entry=0x7fc6a0138df0, addr=addr@entry=156, data=1648892416,
size=size@entry=4, attrs=attrs@entry=...)
8  0x000000000042df17 in address_space_write_continue (...)
10 0x000000000043084d in address_space_rw (...)
11 0x000000000047451b in kvm_cpu_exec (cpu=cpu@entry=0x1ab11b0)
12 0x000000000045dcf5 in qemu_kvm_cpu_thread_fn (arg=0x1ab11b0)
13 0x0000000000870631 in qemu_thread_start (args=args@entry=0x1acfb50)
14 0x00000000008959a7 in thread_entry_for_hotfix (pthread_cb=<optimized out>)
15 0x00007fc6b0a60dd5 in start_thread ()
16 0x00007fc6b078a59d in clone ()

(gdb) f 5
5  0x000000000047897a in memory_region_write_accessor (...)
529	    mr->ops->write(mr->opaque, addr, tmp, size);
(gdb) p /x tmp
$9 = 0x62481a00 <-- last byte 0x00 is @epid

xhci_doorbell_write() already check the upper bound of @slotid an @epid,
it also need to check the lower bound.

Cc: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Longpeng <longpeng2@huawei.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 1556605301-44112-1-git-send-email-longpeng2@huawei.com

[ kraxel: fixed typo in subject line ]

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

hw/usb/hcd-xhci.c

@@ -3137,7 +3137,7 @@ static void xhci_doorbell_write(void *ptr, hwaddr reg,
         streamid = (val >> 16) & 0xffff;
         if (reg > xhci->numslots) {
             DPRINTF("xhci: bad doorbell %d\n", (int)reg);
-        } else if (epid > 31) {
+        } else if (epid == 0 || epid > 31) {
             DPRINTF("xhci: bad doorbell %d write: 0x%x\n",
                     (int)reg, (uint32_t)val);
         } else {