2f41c4e8 exposed some other edge cases as well. Globally resetting the
pass info was not the right way to go about it, because we don't know in
advance what the frame type is going to be - at least not with the
current code structure. (In principle, we could separately indicate the
frame type and the pass type and then only reset it on the first
actual pass_describe call, but that's annoying as well)
Also fixes a latent issue where p->pass was never initialized, which
broke the MP_DBG debugging code in some cases.
69289aec6c