bugfix: cancel sell offer when accept is running increases balance by the offer and not by the reserved

with cancel request:
https://masterchain.info/selloffer.html?tx=fcdbf535e27854b02e4420693814d985c56f4baab19180feac479bcce9367a85&currency=MSC
while the accept was running:
https://masterchain.info/sellaccept.html?tx=8457bdd39838054e6814ba0732a894936deb5fadc08110eebf0269f58057b32e&currency=MSC
the bug showed 19.0 MSC instead of 18.0 MSC on the address:
https://masterchain.info/Address.html?addr=1C7AmZ7sEHNLTWdoU7eUqXDPRuM6kmt5a7&currency=MSC

This extra 1.0 MSC appeared from no-where, so there is no problem to revert this bug.