Fix#140813 crash, when bevel operation doesn't choose proper representative
face (called frep, facerep or rep_face in code). In such scenario a nullptr
would be assigned to uv_face->attached_frep field in register_uv_face function
and later, as a result of that, BM_face_vert_share_loop function would crash
during call to update_uv_vert_map function.
This commit also includes additional safety check in register_uv_face function,
as well as removes compiler warning about assigned but unused center_bme variable.
This is from PR https://projects.blender.org/blender/blender/pulls/140864
but applied to the 4.5 release branch.
515533b342