BUG: Cannot pick empty vline

I tried to create a picker on an empty vline. This seems to not work but I have used this before I think with other lines, so thought that it should. I guess I will just not use vlines for now.
```
In [1]: from matplotlib import pyplot as plt

In [2]: import matplotlib

In [3]: matplotlib.__version__
Out[3]: '1.3.1'

In [4]: vl = plt.vlines([], 0, 1, picker=8)

In [5]: plt.show()  # Now press anywhere into the axis
Floating point exception (core dumped)
```


From GDB:
```
Program received signal SIGFPE, Arithmetic exception.
0x00007fffe48979d0 in _path_module::point_in_path_collection(Py::Tuple const&) ()
   from /usr/lib/pymodules/python2.7/matplotlib/_path.so
```