If you break the internals of better_exec(), you get a very weird
error about tb_next not being a method of None. Fix this by checking
we can step back a trace level.
(Bitbake rev: 1d710ed484f68fca0789022dde7ba877b9a894f5)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>