[Bf-python] Incorrect checking for zero length vector

[Campbell Barton]

Found that AngleBetweenVecs raises an error when checking for a zero 
length Vector

# Heres my test from the interactive console.
print Mathutils.AngleBetweenVecs(Mathutils.Vector(10.0, 0 , 0), 
Mathutils.Vector(0,1,0))
exceptions.AttributeError:  Mathutils.AngleBetweenVecs(): zero-length 
vectors not acceptable

- Cam

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Campbell J Barton

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