Caused by rB3b6ee8cee708: Refactor: Move vertex group names to object data

Above commit was trying to get the vertexgroup from the mesh that is
passed into deformVertsEM (but that can be NULL).
When can it be NULL, when is is non-NULL?
editbmesh_calc_modifiers only passes in a non-NULL mesh to
deformVertsEM under certain conditions:

• a non-deform-only modifier is handled currently
• a non-deform-only modifier preceeds the current modifier
• a deform-only modifier preceeds the current modifier (and the current

one depends on normals)

So the passed-in mesh cannot be relied on, now get the vertex group from
the context object data.

Related commit: rB8f22feefbc20