Fix array subscript warnings

Commit a5cf808 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

Author: danielgustafsson

src/bin/pg_dump/filter.c

@@ -185,14 +185,14 @@ filter_get_keyword(const char **line, int *size)
 	*size = 0;
 
 	/* Skip initial whitespace */
-	while (isspace(*ptr))
+	while (isspace((unsigned char) *ptr))
 		ptr++;
 
-	if (isalpha(*ptr))
+	if (isalpha((unsigned char) *ptr))
 	{
 		result = ptr++;
 
-		while (isalpha(*ptr) || *ptr == '_')
+		while (isalpha((unsigned char) *ptr) || *ptr == '_')
 			ptr++;
 
 		*size = ptr - result;
@@ -301,7 +301,7 @@ read_pattern(FilterStateData *fstate, const char *str, PQExpBuffer pattern)
 	bool		found_space = false;
 
 	/* Skip initial whitespace */
-	while (isspace(*str))
+	while (isspace((unsigned char) *str))
 		str++;
 
 	if (*str == '\0')
@@ -312,7 +312,7 @@ read_pattern(FilterStateData *fstate, const char *str, PQExpBuffer pattern)
 
 	while (*str && *str != '#')
 	{
-		while (*str && !isspace(*str) && !strchr("#,.()\"", *str))
+		while (*str && !isspace((unsigned char) *str) && !strchr("#,.()\"", *str))
 		{
 			/*
 			 * Append space only when it is allowed, and when it was found in
@@ -351,7 +351,7 @@ read_pattern(FilterStateData *fstate, const char *str, PQExpBuffer pattern)
 		found_space = false;
 
 		/* skip ending whitespaces */
-		while (isspace(*str))
+		while (isspace((unsigned char) *str))
 		{
 			found_space = true;
 			str++;
@@ -400,7 +400,7 @@ filter_read_item(FilterStateData *fstate,
 		fstate->lineno++;
 
 		/* Skip initial white spaces */
-		while (isspace(*str))
+		while (isspace((unsigned char) *str))
 			str++;
 
 		/*