accumulate offset for g_base64_decode_step

Commit 6c455d5766a0a5e3bd283c3f8b6b0c68754da013 fixed an infinite loop
for subtitles bigger than ZLIB_CHUNK. However, the offset is only set and not
accumulated with every iteration, therefore still causing the issue for
subtitles bigger than 2 * ZLIB_CHUNK.