The private data of a virStorageSource which is backing an iSCSI hostdev
may be NULL if no authentication is present. The code handling the
hotplug would attempt to extract the authentication info stored in
'secinfo' without checking if it is allocated which resulted in a crash.

Here we opt the easy way to check if srcPriv is not NULL so that we
don't duplicate all the logic which selects whether the disk source has
a secret.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1597550Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>