Fix short read protection logic for querying more rows

Discovered by benedict while reviewing CASSANDRA-13747:

While reviewing I got a little suspicious of the modified line DataResolver :479, as it seemed that n and x were the wrong way around... and, reading the comment of intent directly above, and reproducing the calculation, they are indeed.

This is probably a significant enough bug that it warrants its own ticket for record keeping, though I'm fairly agnostic on that decision.

I'm a little concerned about our current short read behaviour, as right now it seems we should be requesting exactly one row, for any size of under-read, which could mean extremely poor performance in case of large under-reads.

I would suggest that the outer unconditional Math.max is a bad idea, has been (poorly) insulating us from this error, and that we should first be asserting that the calculation yields a value >= 0 before setting to 1.