sodiumHex2Bin silently accepts garbage input

[timmc]

The implementation hexToBytes accepts non-hex characters and does math on their values as if they are hex chars. For example,ABCD__ and ABCDEF turn into the same bytes. (Character.digit uses a return value of -1 as an error code.)

(EDIT: I had previously thought it didn't fail for an odd number of characters, but it actually does. However, an explicit check would be safer.)