Backport Tim's checkin 1.17:

Backport Tim's checkin 1.17:

SF patch #440144:  Tests and minor bugfix for uu module

decode():  While writing tests for uu.py, Nick Mathewson discovered
that the 'Truncated input file' exception could never get raised,
because its "if not str:" test was actually testing the builtin
function "str", not the local string vrbl "s" as intended.