Fix to work with any digest size
Fix to work with any digest size

Offset should be calculated using the last byte of the digest.  Hardcoding for byte #19 only works for 160-bit digests (e.g. sha1). See, e.g. line 163 of https://fedorahosted.org/freeotp/browser/android/app/src/main/java/org/fedorahosted/freeotp/Token.java?rev=e1dbb3c6c3faf112a4bebb75057e0f739c52ac97
Spomky-Labs/otphp