cmd/go: go list crashes when func Example has no body #28044
Description
The underlying cause appears to be an unguarded nil dereference of ast.File.Body in go/doc.Examples.
$ cd $GOPATH/src
$ cat test/example_test.go
package test_test
func Example()
$ go version
go version devel +497fd2c09c Fri Oct 5 17:56:34 2018 +0000 linux/amd64
$ go list -test test
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x55bdb1]
goroutine 1 [running]:
go/ast.(*BlockStmt).Pos(...)
/home/adonovan/go/src/go/ast/ast.go:728
go/doc.lastComment(0x0, 0x0, 0x0, 0x0, 0x558f94, 0xc000105444)
/home/adonovan/go/src/go/doc/example.go:409 +0x11
go/doc.exampleOutput(0x0, 0x0, 0x0, 0x0, 0x1, 0xc000344cb8, 0x4f236d)
/home/adonovan/go/src/go/doc/example.go:108 +0x4d
go/doc.Examples(0xc000344e38, 0x1, 0x1, 0x9, 0x0, 0x4)
/home/adonovan/go/src/go/doc/example.go:75 +0x559
cmd/go/internal/load.(*testFuncs).load(0xc0000ba930, 0xc0004cc0c0, 0x3f, 0x976c2f, 0x6, 0xc0000ba98a, 0xc0000ba98b, 0xc000121010, 0x5c23bb)
/home/adonovan/go/src/cmd/go/internal/load/test.go:530 +0x107
cmd/go/internal/load.loadTestFuncs(0xc0000ab680, 0x7, 0x0, 0x0)
/home/adonovan/go/src/cmd/go/internal/load/test.go:417 +0x257
cmd/go/internal/load.TestPackagesFor(0xc0000ab680, 0x0, 0x1, 0xc00000e640, 0x1, 0x1, 0x21)
/home/adonovan/go/src/cmd/go/internal/load/test.go:234 +0x18dd
cmd/go/internal/list.runList(0xd76880, 0xc00001e0f0, 0x1, 0x1)
/home/adonovan/go/src/cmd/go/internal/list/list.go:449 +0x19a7
main.main()
/home/adonovan/go/src/cmd/go/main.go:219 +0x843