Fix PR#31 -- zfill() mishandles empty string.

Lib/string.py

@@ -475,7 +475,7 @@ def zfill(x, width):
 	n = len(s)
 	if n >= width: return s
 	sign = ''
-	if s[0] in ('-', '+'):
+	if s[:1] in ('-', '+'):
 		sign, s = s[0], s[1:]
 	return sign + '0'*(width-n) + s