Issue 5364: Simplify Drul_array indexing

... so that it is more obvious that the index is never out of bounds

https://sourceforge.net/p/testlilyissues/issues/5364/

I thought that maybe the arithmetic was done for optimization purposes, but even if that was originally the case, Compiler Explorer shows simpler assembly from the simpler comparison: https://godbolt.org/g/cR1zVX

[dak, 2018-07-01 12:18:52]

Well, you basically anticipated all considerations I could have brought up and
dealt with them exhaustively.

LGTM