Blender freezes when exporting with shapekeys

```
Traceback (most recent call last):
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1938, in save
exp.export()
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1848, in export
self.export_scene()
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1497, in export_scene
self.export_node(obj, 2)
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1455, in export_node
self.export_node(x, il)
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1442, in export_node
self.export_mesh_node(node, il)
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1031, in export_mesh_node
meshdata = self.export_mesh(node, armature)
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 416, in export_mesh
md = self.export_mesh(node, None, k, None, shape.name)
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 686, in export_mesh
v.normal.x, v.normal.y, v.normal.z)
KeyboardInterrupt

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/__init__.py", line 159, in execute
return export_dae.save(self, context, **keywords)
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1938, in save
exp.export()
File "/Applications/Blender2.78.app/Contents/Resources/2.78/scripts/addons/io_scene_dae/export_dae.py", line 1933, in __exit__
bpy.data.meshes.remove(mesh)
RuntimeError: Error: Mesh 'Mesh.002' must have zero users to be removed, found 1 (try with do_unlink=True parameter)

```