setting invalid vector swizzle attributes (from blender2.4x docs for instance), could crash blender.

- disallow setting swizzle's that have duplicate values: vec.xxx = [1,2,3]
 - raise an error if the list setting a swizzle's longer.
 - mathutils write callback was running on the wrong vector, which could crash blender.

source/blender/python/doc/examples/mathutils_vector.py

@@ -50,6 +50,6 @@ vec2d[:] = vec3d[:2]
 # Vectors support 'swizzle' operations
 # See http://en.wikipedia.org/wiki/Swizzling_(computer_graphics)
 vec.xyz = vec.zyx
-vec.xy = vec.zw
-vec.xxy = vec.wzz
-vec.yzyz = vec.yxyx
+vec.xy = vec4d.zw
+vec.xyz = vec4d.wzz
+vec4d.wxyz = vec.yxyx