SF bug 431772: traceback.print_exc() causes traceback

Patch from Michael Hundson.
format_exception_only() blew up when trying to report a SyntaxError
from a string input (line is None in this case, but it assumed a string).
Bugfix candidate.

Lib/traceback.py

@@ -171,6 +171,7 @@ def format_exception_only(etype, value):
                 if not filename: filename = "<string>"
                 list.append('  File "%s", line %d\n' %
                             (filename, lineno))
+                if line is not None:
                     i = 0
                     while i < len(line) and line[i].isspace():
                         i = i+1