Fix for expected private key length

We're working with the decimal form of the number here, so it can be slightly variable since any leading zeroes from the hex are not used.  I generated 1,000,000 keys and the range of sizes were:

Array
(
    [0] => 78
    [1] => 77
    [2] => 76
    [3] => 75
    [4] => 74
    [5] => 73
    [6] => 72
)

I think it's safe to say we shouldn't see anything less than 72, although it's still theoretically possible since the number can be in the range of [1, n-1].