Don't waste entropy.

From [DecryptoCat](http://tobtu.com/decryptocat.php),

> They generating a random floating point number by getting 16 random
bytes of data with values less than 250 and converting each of them to a
single decimal digit. This is basically wasting 15 random bytes every
time they generate a random floating point number because they then use
it to generate a character from a small list of characters.