Fixup itertools recipe to cover a corner case.

Doc/library/itertools.rst

@@ -686,6 +686,8 @@ which incur interpreter overhead.
        # number items returned:  (n+r-1)! / r! / (n-1)!
        pool = tuple(iterable)
        n = len(pool)
+       if not n and r:
+           return
        indices = [0] * r
        yield tuple(pool[i] for i in indices)
        while 1:

Lib/test/test_itertools.py

@@ -1268,6 +1268,8 @@ Samuele
 ...     "combinations_with_replacement('ABC', 3) --> AA AB AC BB BC CC"
 ...     pool = tuple(iterable)
 ...     n = len(pool)
+...     if not n and r:
+...         return
 ...     indices = [0] * r
 ...     yield tuple(pool[i] for i in indices)
 ...     while 1: