Fix panic if all players but one fold preflop

[alcamerone]

Hi there,

Thanks for publishing this library, I was impressed by how comprehensive it is!

I found a bug where, if all the players fold pre-flop but one, it panics as the the players' hands are not big enough without any table cards for comparisons in the payout. I solved this by checking if the number of players contesting the pot is greater than one, and, if not, simply awarding the pot to the only contestant. Please see the test case I added as well.

Hope this is helpful, thanks again!

• alcamerone