The root of the issue was that while reading a new blendfile, the
current `G_MAIN` is still the old one, not the one in which new data is
being read.
Since the remapping callback taking care of UI data during ID remapping
is using `G_MAIN`, it is processing the wrong data, so when deleting the
invalid shapekey (from `BLO_main_validate_shapekeys`), the UI data of
the new bmain would not be properly remapped, causing invalid memory
access later when recomputing user counts (calls to
`BKE_main_id_refcount_recompute`).
This is fixed by adding a new `BKE_id_delete_ex` function that takes
extra remapping options parameter, and calling it from
`BLO_main_validate_shapekeys` with extra option to enforce handling of
UI data by remapping code.
NOTE: At some point we have to check if that whole UI-callback thing is
still needed, would be good to get rid of it and systematically process
UI-related ID pointers like any others. Current situation is... fragile
to say the least.
ade83b21d1