unwind.c Only do a linear search if there isn't a search header.

unwind.c Only do a linear search if there isn't a search header.

There always should be one, we create it in the translator
if it didn't exist. Only if we are using elfutils < 0.142
should these ever be missing. If the binary search fails,
either because the unwind data is bad, or the address isn't
covered, don't fall back to linear searching. Add extra
checks and warning about bad debug frame header.