Clear container after move

Summary:
Clear parents after move in
DisconnectedBlockTransactions::addForBlock to ensure a known state
before reusing the variable (the standard enforces a valid state, but
not necessarely a known & specific one).

Signed-off-by: Andres Correa Casablanca <andres@thirdhash.com>

Test Plan: Running the current tests should be enough.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2344