ArrayIndexOutOfBoundsException thrown by keepDice if keep > times

Pointed out by daxx367 on discord:

Calling keep(2,6,4) produces an ArrayIndexOutOfBoundsException because it does not check that keep <= times before calling the dropDice() function.

dicelib/src/main/java/net/rptools/common/expression/function/DiceHelper.java

Line 69 in e837eec

return dropDice(times, sides, times - keep);

The keepLowestDice() function has the same problem:

dicelib/src/main/java/net/rptools/common/expression/function/DiceHelper.java

Line 73 in e837eec

return dropDiceHighest(times, sides, times - keep);

I'll have a PR fixing this shortly.