go/ast: break out after first variable in ExampleCommentMap

The current ExampleCommentMap might panic if there are more satisfied
comments in the parsed program.

Change-Id: Ibe6943470aa0cfb450dae9fc07c1199acaabef73
GitHub-Last-Rev: c79e98c2868179ea47618b296ce28c5eebc0b99d
GitHub-Pull-Request: golang/go#28587
Reviewed-on: https://go-review.googlesource.com/c/147359
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/ast/example_test.go

@@ -176,6 +176,7 @@ func main() {
 		if gen, ok := decl.(*ast.GenDecl); ok && gen.Tok == token.VAR {
 			copy(f.Decls[i:], f.Decls[i+1:])
 			f.Decls = f.Decls[:len(f.Decls)-1]
+			break
 		}
 	}