acl: Fix use after free in qemu_acl_reset()

Reproducer:

    $ MALLOC_PERTURB_=234 qemu-system-x86_64 -vnc :0,acl,sasl [...]
    QEMU 0.15.50 monitor - type 'help' for more information
    (qemu) acl_add vnc.username fred allow
    acl: added rule at position 1
    (qemu) acl_reset vnc.username
    Segmentation fault (core dumped)

Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
