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Fix T89393: crash when selecting edges when geometry nodes has "on cage" turned on

The core problem is that the geometry nodes modifier sometimes support
"mapping" (i.e. it remembers which new vertices correspond to edit mode
vertices) and sometimes it does not, depending on what the nodes are doing.
Also see rB07ce9910f7cc.

The solution here is that the fallback case in `BKE_mesh_foreach_mapped_edge`
does not call the callback with "invalid" indices.

Differential Revision: https://developer.blender.org/D12007