pgsql: Fix unportable use of isxdigit() with char (rather than unsigned

Log Message:
-----------
Fix unportable use of isxdigit() with char (rather than unsigned char)
argument, per warnings from buildfarm member pika. Also clean up code
formatting a trifle.

Modified Files:
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pgsql/src/backend/parser:
scan.l (r1.165 -> r1.166)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/scan.l?r1=1.165&r2=1.166)