Corrupt/invalid PDF and EPS files when saving a logscaled plot made with negative values

This is with matplotlib 1.1.0, via the Pylab interface (I'm using a version supplied with PythonXY):

```
import pylab as p
import numpy as np

x = np.arange(10)
y = np.arange(10)-1

p.scatter(x,y)
p.xscale('log')
p.yscale('log')
```

Saving the plot as an EPS or PDF produces an invalid file; when opening the PDF, Adobe Reader says "Path lacks initial MOVETO".  PNG output looks fine.

Granted, negative values can't be plotted on a log scale, but creating a bad output file seems like a bug.  As a workaround, I now filter my data for negative values before plotting.