* base64-decode.c (base64_decode_single): Return -1 on error.

Also keep track of the number of padding characters ('=') seen.
(base64_decode_update): New argument dst_length. Return -1 on error.
(base64_decode_status):  Renamed function...
(base64_decode_final): ... to this.

* base64.h (struct base64_decode_ctx): Deleted STATUS attribute.
Added PADDING attribute.

Rev: src/nettle/base64-decode.c:1.4
Rev: src/nettle/base64.h:1.11