pgsql: Fix array subscript warnings

Fix array subscript warnings

Commit a5cf808be55 accidentally passed signed chars to isalpha and
isspace in the parser code which leads to undefined behavior.  Fix
by casting the parameters to unsigned chars.

Author: Pavel Stehule <pavel.stehule@gmail.com>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/388186.1701315586@sss.pgh.pa.us
Discussion: https://postgr.es/m/ZWgg5xim2CXQcfmh@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/17935e1fdf0aff083347b4b2d6268d945041d188

Modified Files
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src/bin/pg_dump/filter.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)