Fix unportable usage of <ctype.h> functions.

Fix unportable usage of <ctype.h> functions.

isdigit(), isspace(), etc are likely to give surprising results if passed a
signed char.  We should always cast the argument to unsigned char to avoid
that.  Error in commit 63d6b97fd, found by buildfarm member gaur.
Back-patch to 9.3, like that commit.