fix tryLocateTx for masterchain dest

[Lapo4kaKek]

The issue was that for masterchain blocks the client still attempted to fetch shards via get_shards, then substituted a shard from that list while keeping workchain = -1 and the masterchain seqno.
As a result, listBlockTransactions received an invalid block ID (masterchain workchain + shard from a regular workchain + masterchain seqno), and the liteserver returned
LITE_SERVER_UNKNOWN: invalid block id requested.

This fix detects masterchain destinations (workchain == -1) and uses the masterchain shard directly instead of calling get_shards.