lib: fix ZUC-EIA3

When generating keystream to authenticate multiple buffers,
only 8 bytes more of keystream are needed when all data
to authenticate has been processed, so only 8 more bytes
were generated for all buffers.
The problem here is that this only works if all buffers
have the same size. If the other buffers have different
sizes, they will need 16 bytes to be generated before
getting into the single buffer code path.