Zero length `\pgfpathclose` segments with non-zero angle are rendered incorrectly

[jonschz]

This is a more general case of the issue discussed in #13.

\begin{tikzpicture}
  \draw[draw=green,opacity=0.5, double distance=5pt, save path=\tmppath] 
    (0,0) arc[start angle=270, end angle=360, radius=1cm] arc[start angle=90, end angle=180, radius=1cm] -- cycle;
  \path[draw=purple, opacity=0.5, double distance=5pt, nfold=4, use path=\tmppath];
\end{tikzpicture}