the uninitialized cumofs was leading to random sizes for
the memset when initializing local structures, potentially
leading to segfaults from it. Only a problem with GNU
designated initializers, which we didn't test very well.
See testcase.
anonymous struct members were somewhat broken as the testcase
demonstrates. The reason is the jumping through hoops to fiddle
with the offsets I once introduced to avoid having to track
a cumulative offset. That's now not necessary anymore and actively
harmful, doing the obvious thing is now better.
See the added testcase. When one used designators like .a.x to initialize
sub-members of members, and didn't then initialize all of them the
required zero-initialization of the other sub-members wasn't done.
The fix also enables tiny code cleanups.
