terminal: Don't crash on non-ascii chars

isalpha() and isdigit() don't like huge values.
Noticed with ascii-art in the shell prompt.

clients/terminal.c

@@ -2268,6 +2268,9 @@ static int wordsep(int ch)
 {
 	const char extra[] = "-,./?%&#:_=+@~";
 
+	if (ch > 127)
+		return 1;
+
 	return ch == 0 || !(isalpha(ch) || isdigit(ch) || strchr(extra, ch));
 }