chain: fix check_chains() (Fixes #33)

Bugs cause segv on startup.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
block_info: remove unused prevs field.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
recv_block: ask for block contents on reinject.

Now we query blocks out of order, we often will get a block through
reinjection (once we finally get its prev) rather than directly from
the packet path.  Assume that if this happens, we are syncing, and thus
want to ask about contents and children.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>