Fixed segfault caused by empty polygon.
Fixed segfault caused by empty polygon.

svn path=/trunk/libavg/; revision=4517

hoodie/libavg,th3infinity/libavg,scotty007/libavg,libavg/libavg,libavg/libavg,scotty007/libavg,yyd01245/libavg,libavg/libavg,scotty007/libavg,yyd01245/libavg,hoodie/libavg,hoodie/libavg,th3infinity/libavg,hoodie/libavg,libavg/libavg,th3infinity/libavg,th3infinity/libavg,scotty007/libavg,yyd01245/libavg,yyd01245/libavg