fix handler use-after-free

fix handler use-after-free

The problem here is that __lxc_start frees the handler, so any use
afterwards is invalid. Since we don't have access to the actual struct
lxc_container object in __lxc_start, let's pass a pointer to error_num in
so it can be returned.

Unfortunately, I'm a little too paranoid to change the return type of
lxc_start, since it returns failure if some of the cleanup fails, which
may be useful in some cases. So let's keep this out of band.

Closes #2218
Closes #2219

Reported-by: Felix Abecassis <fabecassis@nvidia.com>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>