This modifier only uses mesh to get vgroup, which is only needed in case
modified object is indeed a mesh! Building a mesh from curve here is not
only useless and time-consuming, it will also easily fail the assert
about same number of vertices!
Note that surface_project and subsurf option also need more work at some
point, but this is probably not that urgent for now.
Also, use MOD_get_vgroup() helper in modifier code itself and pass
resulting MDeformVert & index to BKE_shrinkwrap's `shrinkwrapModifier_deform()`,
this is simpler and avoids duplicating vgroup handling code.
Related to T57972.
15add11595