SGWireframeWidget: add some needed checks.

While drawing highlighted faces, it could happen that some
uninitialized variables were accessed, while it wouldn't make sense to
draw a face at all. Now i added the necessary check to not try to draw
the face in that case.
SGWireframeWidget: Reset geometry size on change.

When the geometry changes, that the SGWireframeWidget displays, the
variables holding the size of it wouldn't get reset correctly, causing
the zoom to be wrong after changing the geometry node. This commit
fixes that.