Fix crash when re-assigning groups

Fix crash when re-assigning groups

This works around an issue where gtk sends two callbacks when a radio
selection changes. First: unselect, then select new item.

Previously:
 Two tracks, two groups. Select all tracks, assign to 2nd group. Crash.

The first callback, "unselect" iterates over all tracks, removes them
from any groups. Eventually all groups are unused and hence destroyed.

The 2nd callback - "select" - now uses a just destroyed group.