Consider sending an array of preLen sizes to StructureSupport classes for sanity checks

[jesse-gallagher]

This is essentially an expansion of #333

The trouble here is that there's a reasonable chance that one "Len" component of a structure may be corrupt - seen in practice as presumably a negative value. Our current idiom is to add all these values and then pass them to StructureSupport, but doing so means that we have no way to detect likely corruption. Naturally, we can't detect all corruption: if the value is 3 but should be 2 or 4, there's no real way to tell what's corrupt, but we COULD see if an individual component is negative or larger than the total size. This would cover for the plausible case of the structure being created with uninitialized memory and then only some "Len" components set.