block/iscsi: fix initialization of iTask in iscsi_co_get_block_status

in case of unaligned requests or on a target that does not support
block provisioning we leave iTask uninitialized and check iTask.task
for NULL later.

Fixes: e38bc234
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1515425247-21730-1-git-send-email-pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>