internal/buildinfo: avoid panic on nil symbol for elf

For very old binaries or binaries built with forks of Go, looking up
an elf symbol that does not exists might not return an error, yet
just a nil symbol, resulting in a panic. We hence make an additional
check. This is also consistent with other binary formats.

Change-Id: I488eeca4ada27a0be48c7da82ca7b1391a7ce394
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/585336
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>

internal/buildinfo/additions_buildinfo.go

@@ -26,7 +26,7 @@ var ErrNoSymbols = errors.New("no symbol section")
 // SymbolInfo is derived from cmd/internal/objfile/elf.go:symbols, symbolData.
 func (x *elfExe) SymbolInfo(name string) (uint64, uint64, io.ReaderAt, error) {
 	sym, err := x.lookupSymbol(name)
-	if err != nil {
+	if err != nil || sym == nil {
 		if errors.Is(err, elf.ErrNoSymbols) {
 			return 0, 0, nil, ErrNoSymbols
 		}