xhci: Fix memory leak in xhci_address_slot

xhci: Fix memory leak in xhci_address_slot

Address Sanitizer shows memory leak in xhci_address_slot
hw/usb/hcd-xhci.c:2156 and the stack is as bellow:

Direct leak of 64 byte(s) in 4 object(s) allocated from:
    #0 0xffff91c6f5ab in realloc (/lib64/libasan.so.4+0xd35ab)
    #1 0xffff91987243 in g_realloc (/lib64/libglib-2.0.so.0+0x57243)
    #2 0xaaaab0b26a1f in qemu_iovec_add util/iov.c:296
    #3 0xaaaab07e5ce3 in xhci_address_slot hw/usb/hcd-xhci.c:2156
    #4 0xaaaab07e5ce3 in xhci_process_commands hw/usb/hcd-xhci.c:2493
    #5 0xaaaab00058d7 in memory_region_write_accessor qemu/memory.c:507
    #6 0xaaaab0000d87 in access_with_adjusted_size memory.c:573
    #7 0xaaaab000abcf in memory_region_dispatch_write memory.c:1516
    #8 0xaaaaaff59947 in flatview_write_continue exec.c:3367
    #9 0xaaaaaff59c33 in flatview_write exec.c:3406
    #10 0xaaaaaff63b3b in address_space_write exec.c:3496
    #11 0xaaaab002f263 in kvm_cpu_exec accel/kvm/kvm-all.c:2288
    #12 0xaaaaaffee427 in qemu_kvm_cpu_thread_fn cpus.c:1290
    #13 0xaaaab0b1a943 in qemu_thread_start util/qemu-thread-posix.c:502
    #14 0xffff908ce8bb in start_thread (/lib64/libpthread.so.0+0x78bb)
    #15 0xffff908165cb in thread_start (/lib64/libc.so.6+0xd55cb)

Cc: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Ying Fang <fangying1@huawei.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-id: 20190827080209.2365-1-fangying1@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>