pgsql: Fix a portability bug (ye olde not casting a argument

Log Message:
-----------
Fix a portability bug (ye olde not casting a <ctype.h> argument to
unsigned char).  Fortunately we still have buildfarm machines that
will flag this.  Seems to be new in CVS HEAD, so no backpatch.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        cash.c (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/cash.c.diff?r1=1.70&r2=1.71)