Fix #37153: Bool union of 2 planes makes Blender 2.69 RC2 hang

Fix deadlock in Carve when rescaling to zero scale.
basically, scaling to zero scale is not what we want :)

Boolean result could still be unpredictable coz plane is
not a closed manifold.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60867 954f8c5b-7b00-dc11-b283-0030488c597c