debug/buildinfo: avoid nil defererence for invalid XCOFF

I've made it return 0 following what the other DataStart implementation
do when they do not found the section.

Fixes #52718

Change-Id: I44292fed15bb21b9aba712ad15ed74e0ce231b5a
GitHub-Last-Rev: 081a35e
GitHub-Pull-Request: #52722
Reviewed-on: https://go-review.googlesource.com/c/go/+/404254
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/debug/buildinfo/buildinfo.go

@@ -393,5 +393,8 @@ func (x *xcoffExe) ReadData(addr, size uint64) ([]byte, error) {
 }
 
 func (x *xcoffExe) DataStart() uint64 {
-	return x.f.SectionByType(xcoff.STYP_DATA).VirtualAddress
+	if s := x.f.SectionByType(xcoff.STYP_DATA); s != nil {
+		return s.VirtualAddress
+	}
+	return 0
 }