Fix Cygwin build warnings

xB.c

@@ -1009,7 +1009,7 @@ is_valid_plugin_name (const char *name)
 	if (!*name)
 		return false;
 	for (const char *p = name; *p; p++)
-		if (!isgraph (*p) || *p == '/')
+		if (!isgraph ((uint8_t) *p) || *p == '/')
 			return false;
 	return true;
 }
@@ -1213,7 +1213,7 @@ parse_bot_command (const char *s, const char *command, const char **following)
 	s += command_len;
 
 	// Expect a word boundary, so that we don't respond to invalid things
-	if (isalnum (*s))
+	if (isalnum ((uint8_t) *s))
 		return false;
 
 	// Ignore any initial spaces; the rest is the command's argument