When writing to the qemu-pr-helper socket failed, the persistent
reservation manager was correctly disconnecting the socket, but it
did not clear pr_mgr->ioc.  So the rest of the code did not know
that the socket had been disconnected, accessed pr_mgr->ioc and
happily caused a crash.

To reproduce, it is enough to stop qemu-pr-helper between QEMU
startup and executing e.g. sg_persist -k /dev/sdb.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>