pr-manager-helper: avoid SIGSEGV when writing to the socket fail

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>

scsi/pr-manager-helper.c

@@ -71,6 +71,7 @@ static int pr_manager_helper_write(PRManagerHelper *pr_mgr,
         if (n_written <= 0) {
             assert(n_written != QIO_CHANNEL_ERR_BLOCK);
             object_unref(OBJECT(pr_mgr->ioc));
+            pr_mgr->ioc = NULL;
             return n_written < 0 ? -EINVAL : 0;
         }