Was not allocating space for the frame above sigpanic,
nor was it pushing the LR into the right place.
Because traceback past sigpanic only needs the
LR for faulting leaves, this was not noticed too much.
But it did break the sync/atomic nil deref tests.

Change-Id: Icba53fffa193423aab744c37f21ee893ce2ee3ac
Reviewed-on: https://go-review.googlesource.com/12926Reviewed-by: David Crawshaw <crawshaw@golang.org>