talloc_stack: report lazy freeing (panic if DEVELOPER).

talloc_stack: report lazy freeing (panic if DEVELOPER).

talloc_stackframe() stacks, so if you forget to free one, the outer
one will free it.  However, it's not a good idea to rely too heavily
on this behaviour: it can lead to delays in the release of memory or
destructors.

I had an elaborate hack to make sure every talloc_stackframe() was
freed in the exact same function it was allocated, however all bugs it
caught were simply lazy freeing, so this patch just checks for that.

This doesn't check for stackframes we don't free up on exit: that would
be nice, but uncovers some uncomfortable (but probably harmless) cases.

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