[bugfix] update lockpoints correctly during reorg

During a reorg, we re-check timelocks on all mempool entries using
CheckSequenceLocks(useExistingLockPoints=false) and remove any
now-invalid entries. CheckSequenceLocks() also mutates the LockPoints
passed in, and we update valid entries' LockPoints using
update_lock_points. Thus, update_lock_points(lp) needs to be called
right after CheckSequenceLocks(lp), otherwise we lose the data in lp.
commit bedf246 introduced a bug by separating those two loops.

src/validation.cpp

@@ -378,6 +378,8 @@ void CChainState::MaybeUpdateMempoolForReorg(
                 }
             }
         }
+        // CheckSequenceLocks updates lp. Update the mempool entry LockPoints.
+        if (!validLP) m_mempool->mapTx.modify(it, update_lock_points(lp));
         return should_remove;
     };