debug/buildid: treat too large string as "not a Go executable"

debug/buildid: treat too large string as "not a Go executable"

If the length does not fit in int, saferio.ReadDataAt returns
io.ErrUnexpectedEOF. Treat is as an invalid format.

Fixes #68692.
For #68592.

Cq-Include-Trybots: luci.golang.try:gotip-linux-386-longtest
Change-Id: Ie856f29c907fd10e6d9b7dfbb6f0d8008a75a1c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/602435
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>