Use `BLI_ghash_insert` instead of `BLI_ghash_reinsert` to store images.
VSE links stored cache keys for each frame in style of linked list.
These links must be maintained when any image/key is removed from the
cache. Reinserting can free key without proper relinking, which would
lead to accessing this freed key when cache limiting frees images in
a frame.
This should not happen since reinserting is prevented in
`seq_cache_put()`, but it's safer to assert, that key is not stored in
hash instead.
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