Fix a compile warning on ARM.

Fix a compile warning on ARM.

Aapo Rantalainen points out that comparing 'char c' against zero gives
rise to gcc's "comparison is always false" warning, which escalates to
an error under -Werror.

This is one of those situations where the warning is doing more harm
than good, but here's a rephrasing which casts to unsigned so that
both negative numbers and positive out-of-range ones can be caught by
the same comparison.