fix/xemm decimal invalid operation error when too much quote balance

Cross Exchange market making returns a "Decimal Invalid Operation" error when quote balance is too high. This is due to "taker_market.c_get_price_for_quote_volume()" being called with quote balance, and this function returns NaN when quote balance is too high.

This bug can be replicated by using paper trade. Setting balance paper to USDT 1000000, and running xemm on an illiquid market (thin order book).

Made slight changes in the code that would prevent this error from popping up. Does not affect the main logic of xemm in any way.