This was caused by r.57812
There were two problems here:
1) vertex_group_vert_select_unlocked_poll() had faulty logic which meant that
it always failed when there were no vgroups present yet - the final return
always just fell through
2) Since the "Assign to New Groups" option was actually implemented using the
same operator as "Assign to Active Group" (just with an extra parameter set), if
the active group was locked, it was not possible to "Assign to New Group" (even
though a new group would not be locked).
4f3f95751a