Fix crash with some doctest syntax errors

I added a test to show the issue, as a separate commit from the fix so you can chose a better fix if there is one.

It would crash like that:

```
Traceback (most recent call last):
  File "./bin/pyflakes", line 3, in <module>
    main()
  File "pyflakes/pyflakes/api.py", line 128, in main
    warnings = checkRecursive(args, reporter)
  File "pyflakes/pyflakes/api.py", line 119, in checkRecursive   
    warnings += checkPath(sourcePath, reporter)
  File "pyflakes/pyflakes/api.py", line 86, in checkPath
    return check(codestr, filename, reporter)
  File "pyflakes/pyflakes/api.py", line 58, in check
    w = checker.Checker(tree, filename)
  File "pyflakes/pyflakes/checker.py", line 254, in __init__
    self.runDeferred(self._deferredFunctions)
  File "pyflakes/pyflakes/checker.py", line 291, in runDeferred  
    handler()
  File "pyflakes/pyflakes/checker.py", line 704, in <lambda>
    self.deferFunction(lambda: self.handleDoctests(node))
  File "pyflakes/pyflakes/checker.py", line 581, in handleDoctests
    example.indent + 4 + e.offset)
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
```