Fix bad check for incomplete polygons, fix div-by-zero.

When only one polyline was left, it would erroneously not be pushed
onto the "incomplete" list.

Also, a vector normalization had no check for division-by-zero, causing
a NaN if the vector length was zero.

Change-Id: I36a9b7a3561e6a490c795806e1b35c10de44a446