Refactor subblock iterators.

Refactor subblock iterators.

Always initialize iterators.

Try to avoid pointer arithmetics on NULL; in other words, move all pointer
arithmetics inside g_isi_sb_iter_init().

There are 4 different ways for representing sub blocks in ISI message:
- 8-bit sub_blocks count, 8-bit sub_block_id/sub_block_len
- 16-bit sub_blocks count, 8-bit sub_block_id/sub_block_len
- 8-bit sub_blocks count, 16-bit sub_block_id/sub_block_len
- 16-bit sub_blocks count, 16-bit sub_block_id/sub_block_len

The compact form g_isi_sb_iter_init() supports 8-bit sub_block count before
start of the sub blocks themselves and 8-bit sub_block_id and sub_block_len.
The full form g_isi_sb_iter_init_full() with explicit longhdr and sub_block
count supports all other cases.