Fix bug that assumes toByteArrayUnsigned() always returns 32 bytes.

To reproduce this bug, try entering the following XPRIV:
xprv9s21ZrQH143K3GMYVMZBZCGProVPNqGXRXz3ktUYt1rXG813HhysPLeNj8AfahpW2d4Tc678FMYndMrw9sqUFuLD2mug9pwojMuNusRbsv9
and use the following derivation path: m/44'

The correct derived private key should be:
xprv9tzN9JZpuT1wiz7y2qsBnjttAxvUQcdjCCEVXdMQmp32FeX4vGve5PA7yFan15Jo71HrxsA6VAnpqWW2JNoaXaLe3NeqWKoYS76AEBfioN7
but the code was previously outputting:
DeaWiSNc9gKmPJgBaytskvDH3Cz4tebKSjxu9t2hSyGVx1gt2xdGPVXjPHoUQd9LBRpcJQDbFdQdEAzqWq2eVeErNLsGwEEWGEGgArZjHap4aP

This was as a result of line 185 in bip32.js calling
this.eckey.priv.toByteArrayUnsigned() without verifying that the
returned byte array had the correct length required for serialization
(32 bytes).

Furthermore, errors would cascade down the path dangerously unnoticed.
For example, using the path m/44'/0' with the XPRIV mentioned above,
the correct derived private key should be:
xprv9xUGaEZ5EBM1tGE9Ujp2EaPC85Pt8PRwMSmk53rvy3YmJbg6jBZM3mybraxFBWFgzrfhEhiiugDMArUqiGDSQF3zHL8wY6AZBXE77Jq2utJ
however the original code would give:
xprv9xUGaEZ5EBM1ueG3H1yNEgQuk9ivpzaHhVbp58hcCJSyrinvZC3k6uvzobs6Av5UmLvjfRQK6AWq2fL9ej27wjr3BURJFddYzvovFYngqLS

This was quite dangerous as there is nothing obviously wrong with the
output. This cascading bug was due to line 254 in bip32.js which also
incorrectly assumed that toByteArrayUnsigned() always returns a 32 byte
long array.