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

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: Michal Privoznik <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>