prev[sink] == 0 means there was a path found with the last step being a
forward edge to the sink, and the edge being at index 0 in the array.
This is a valid path (the same as any other path indicated by prev[sink]
> 0).
maxflow.c. Also in this checkin, fixes to the OS X and GTK back ends
to get ALIGN_VNORMAL right. This is the first time I've used it! :-)
[originally from svn r6390]
