xs_version_bootcheck() must use mortals, as {new,upg}_version() can croak.

xs_version_bootcheck() must use mortals, as {new,upg}_version() can croak.

It's unlikely that XS_VERSION will contain a bogus version string (for long),
but the value passed in (or derived from $XS_VERSION or $VERSION) might well.
For that case, without this change, temporary SVs created within
xs_version_bootcheck() won't be freed (before interpreter exit).