Blender Git Loki

Fix T66631: Crash when converting objects from Curve to Mesh

When `BKE_mesh_new_from_object()` cannot convert an object to a mesh, it
returns `NULL`. This case was not handled at all in
`BKE_mesh_new_from_object_to_bmain()` or `curvetomesh()`, causing a
segmentation fault.

This commit fixes the segmentation fault, and leaves the curve object as
a curve object.

Reviewed By: mont29, brecht, sergey

Differential Revision: https://developer.blender.org/D5217