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 <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic

src/debug/buildinfo/buildinfo.go

@@ -295,6 +295,10 @@ func decodeString(x exe, addr uint64) (string, uint64, error) {
 
 	b, err = readData(x, addr, length)
 	if err != nil {
+		if err == io.ErrUnexpectedEOF {
+			// Length too large to allocate. Clearly bogus value.
+			return "", 0, errNotGoExe
+		}
 		return "", 0, err
 	}
 	if uint64(len(b)) < length {