cmd/link: crash with plugin with no exports

[aclements]

Please answer these questions before submitting your issue. Thanks!

What did you do?

echo "package main; func init() { println("hi") }" > x.go
go build -buildmode=plugin x.go

What did you expect to see?

My very first plugin.

What did you see instead?

# command-line-arguments
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xd8 pc=0x506b24]

goroutine 1 [running]:
panic(0x5b9ea0, 0x6b3ca0)
	/home/austin/go.dev/src/runtime/panic.go:531 +0x1cf
cmd/link/internal/ld.(*deadcodepass).init(0xc420071d20)
	/home/austin/go.dev/src/cmd/link/internal/ld/deadcode.go:255 +0x744
cmd/link/internal/ld.deadcode(0xc4204a2000)
	/home/austin/go.dev/src/cmd/link/internal/ld/deadcode.go:61 +0xca
cmd/link/internal/ld.Main()
	/home/austin/go.dev/src/cmd/link/internal/ld/main.go:186 +0x820
main.main()
	/home/austin/go.dev/src/cmd/link/main.go:55 +0x139

Specifically, this seems to be because there are no exported symbols in my plugin. If I add a dummy exported symbol, the linker does not crash.

System details

go version devel +f135106 Sun Oct 30 19:14:08 2016 +0000 linux/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/austin/r/go"
GORACE=""
GOROOT="/home/austin/go.dev"
GOTOOLDIR="/home/austin/go.dev/pkg/tool/linux_amd64"
TERM="dumb"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build901715014=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
uname -sr: Linux 4.2.0-42-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04.5 LTS
Release:	14.04
Codename:	trusty
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu EGLIBC 2.19-0ubuntu6.9) stable release version 2.19, by Roland McGrath et al.
gdb --version: GNU gdb (GDB) 7.9-gg19

[gopherbot]

CL https://golang.org/cl/32437 mentions this issue.