diskmanage: get_partnum: fix check

diskmanage: get_partnum: fix check

Not replacing it with return, because the current behavior is dying:
    Can't "next" outside a loop block
and the single existing caller in pve-manager's API2/Ceph/OSD.pm does not check
the return value.

Also check for $st, which can be undefined in case a non-existing path was
provided. This also led to dying previously:
    Can't call method "mode" on an undefined value

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>