Bug#961444: Fix for grcompiler FTBFS on architectures where char is unsigned

[Tim Eves <tim_eves@sil.org>]

I've reworked the assert to be explicit about checking the integer range could fit in a signed 8 bit value in commit https://github.com/silnrsi/grcompiler/commit/a60a04691c42f3f99543c83f9911c84e44a268fe

Hopefully this will fix the issue and it isn't really managing to generate an offset outside the -128 <->127 range.

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Tim Eves

Writing Systems Technology - SIL International