Fix: cancel-at-price where balance < amount-to-cancel

This is a fix for the case where a cancel-at-price operation has no effect,
if the available balance is less than the amount-to-cancel. (issue 217)

The commit was tested and is side effect free.
Merge pull request #222 from dexX7/mcore-fix-cancel-at-balance-constraint

Fix: cancel-at-price where balance < amount-to-cancel