base64: properly handle zero length input during decode
base64: properly handle zero length input during decode

Without this explicit check we were accessing memory out-of-bounds (at index
-1).

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc