Guard against potential out-of-bounds lookups in base cell tables

[nrabinowitz]

The functions in baseCells.c should guard against invalid base cell numbers that could result in out-of-bounds reads on the lookup tables.

[alexey-milovidov]

For zero-cost implementation (safe but implementation specific behaviour on out-of-bounds lookup) we can simply extend the arrays to 128 elements (filling the rest with zeros).