Correct segfault introduced by 432c7ea78781a4ba084a1f00e5c034f30ded6cd0.

Closes #31.
Fix possible crash or soft deadlock if active group device rudely detached:

3cdf72aa86f6a607f1a4e333da4141b127570ec8 introduced a possible crash that can occur when a "rude" (i.e. while device is still up/active on a live/running system) detach or exit is done for a grouped device (such as QETH) due to 'dev->group' not being passed to device handler's close function.

The checks 'sysblk.shutdown' are to address a possible soft deadlock that can also potentially occur in the same situation but are unrelated to the 3cdf72aa86f6a607f1a4e333da4141b127570ec8 change.