dgraph: add dgraph_check and CCAN_DGRAPH_DEBUG

This is good form for datastructures, so caller can track weird bugs.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
ccanlint: fix stack smash.

"engla" on IRC reported that ccanlint on linux/powerpc would loop
infinitely calling dep_failed after all tests are run and the score is
output.

Reproduced, and discovered that cannot_run() takes a container_of(),
except our top node is not a struct ccanlint.  The result was harmless on
x86, but set dep_failed to the return address on powerpc, causing that
to be called repeatedly.

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