txdb: Fix assert crash in new UTXO set cursor
txdb: Fix assert crash in new UTXO set cursor

Remove the mistaken assumption that GetKey returning false signifies
an internal database issue. It will return false when the key cannot
be deserialized into the (char,uint256) stanza, which indicates
that the cursor has reached a different kind of key.

Fixes bug #7890 introduced in #7756.

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