Update LootBox.sol: Some Enhancements

[0xScratch]

Basically, this PR works on two changes:

• For Loops Improvement: Here, uint256 i = 0; is changed to uint256 i; cuz the default values of unsigned integers are usually 0. Also, changing i++ to ++i is a good one to decrease the gas costs. Refer this -> Here we haven't used the unchecked block cuz the pragma version is less than 0.8.0
• Using internal function for the modifier: Whenever a modifier is used a lot of times inside a contract, it's better to create an internal function regarding it. This avoids the copying of same code again and again. These two links will help -> 1st and 2nd

Thanks! @asselstine @aodhgan