Update ctype macro usage for platforms with signed chars (Issue #1623)

Author: michaelrsweet

CHANGES.md

@@ -12,6 +12,7 @@ Changes in CUPS v2.4.20 (YYYY-MM-DD)
 - Increased the size of the SNMP supply name buffer used by the network backends
   (Issue #1604)
 - The USB backend now clears a halt on USB errors (Issue #1606)
+- Updated a few character tests for signed char platforms (Issue #1623)
 - Fixed mapping of standard PPD/PWG/legacy media size names to the local PPD
   size name (Issue #1375)
 - Fixed handling of multiple PPD: keywords from filters (Issue #1562,

scheduler/client.c

@@ -773,7 +773,7 @@ cupsdReadClient(cupsd_client_t *con)	/* I - Client to read from */
 	      _cups_strcasecmp(hostname, ServerName) &&
 	      _cups_strcasecmp(hostname, "localhost") &&
 	      !cupsArrayFind(ServerAlias, hostname) &&
-	      !isdigit(hostname[0]) && hostname[0] != '[')
+	      !isdigit(hostname[0] & 255) && hostname[0] != '[')
 	  {
 	   /*
 	    * Nope, we don't do proxies...
@@ -2673,18 +2673,18 @@ check_if_modified(
 
   while (*ptr != '\0')
   {
-    while (isspace(*ptr) || *ptr == ';')
+    while (isspace(*ptr & 255) || *ptr == ';')
       ptr ++;
 
     if (_cups_strncasecmp(ptr, "length=", 7) == 0)
     {
       ptr += 7;
       size = strtoll(ptr, NULL, 10);
 
-      while (isdigit(*ptr))
+      while (isdigit(*ptr & 255))
         ptr ++;
     }
-    else if (isalpha(*ptr))
+    else if (isalpha(*ptr & 255))
     {
       date = httpGetDateTime(ptr);
       while (*ptr != '\0' && *ptr != ';')