Assertion failure `mr != NULL' failed through usb-ehci

Hello,

Using hypervisor fuzzer, hyfuzz, I found an assertion failure through usb-ehci.

This was found in version 5.0.1 (stable-5.0).

--------

qemu-system-i386: src/qemu-repro/exec.c:3581: address_space_unmap: Assertion `mr != NULL' failed.
[1]    14721 abort      src/qemu-repro/build/i386-softmmu/qemu-system-i386


To reproduce the assertion failure, please run the QEMU with following command line.

```
$ qemu-system-i386 -drive file=./hyfuzz.img,index=0,media=disk,format=raw -m 512 -drive if=none,id=stick,file=./usbdisk.img -device usb-ehci,id=ehci -device usb-storage,bus=ehci.0,drive=stick
```