Minor strengthening of asserts.

Minor strengthening of asserts.

While investigating the issue fixed by previous commit, found lots of
segmentation faults due to wrong memory access. Some of them could be
fixed by properly avoiding some computations for dubious memory. To at
least inoculate from other such mishaps, changed some of the obvious
asserts to fatal asserts in some of the affected files.

It is clear that this is a drop in an ocean, as there are lots of
asserts that obviously should be made fatal, and even more asserts for
which there is no fast way of knowing if they need to be fatal
(additional investigation required). Yet I'm just fixing the ones I've
seen (and all obvious in the files I've touched) instead of just letting
it pass, as long as I've been made aware of them.