go/doc: skip examples with no body

Fixes #28044

Change-Id: I0052e078dd34dc3546204416bcc5a99e3146c535
Reviewed-on: https://go-review.googlesource.com/c/140317
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/doc/example.go

@@ -68,6 +68,9 @@ func Examples(files ...*ast.File) []*Example {
 			if !isTest(name, "Example") {
 				continue
 			}
+			if f.Body == nil { // ast.File.Body nil dereference (see issue 28044)
+				continue
+			}
 			var doc string
 			if f.Doc != nil {
 				doc = f.Doc.Text()