fix: out of bounds access in ListFlaggedPieces

[jmg-duarte]

As stated in the title.

The following payload leads to a panic, but doesn't crash the server.

{id:1, jsonrpc:"2.0", method:"boostddata.FlaggedPiecesList", params: [{}, "0001-01-01T00:00:00Z", 0, -1]}