Bug fix: keep the balance from going negative

Sometimes, due to the loss of precision in doubles and floats in case the
daily budget was balanced - the balance might have still been a very small
negative number which caused the balance overdraft button to pop-up. The
solution is more of a hack for right now - it doesn't compensate the
balance, but rather puts a zero in case it is supposed to be zero. This
issue will be addressed in the later versions.