Please answer these questions before submitting your issue. Thanks!

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

package main

import (
    "encoding/asn1"
    "math/big"
)

var B *big.Int

func main() {
    asn1.Marshal(B)
}

What did you expect to see?

no panic.

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x9ab8a]

goroutine 1 [running]:
panic(0xb9280, 0x13d250)
/Users/hiro/go/src/runtime/panic.go:527 +0x1a0
encoding/asn1.makeBigInt(0x0, 0x0, 0x16)
/Users/hiro/go/src/encoding/asn1/marshal.go:154 +0x3a
encoding/asn1.makeBody(0xcf640, 0x0, 0x16, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1e6038, ...)
/Users/hiro/go/src/encoding/asn1/marshal.go:412 +0x1301
encoding/asn1.makeField(0xcf640, 0x0, 0x16, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/Users/hiro/go/src/encoding/asn1/marshal.go:596 +0x2b4
encoding/asn1.Marshal(0xcf640, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc4200001a0)
/Users/hiro/go/src/encoding/asn1/marshal.go:641 +0xe6
main.main()
/Users/hiro/a.go:15 +0x39
exit status 2

Does this issue reproduce with the latest release (go1.7.1)?

System details

go version devel +05cbf45 Sun Oct 16 13:47:13 2016 +0900 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hiro/.go"
GORACE=""
GOROOT="/Users/hiro/go"
GOTOOLDIR="/Users/hiro/go/pkg/tool/darwin_amd64"
TERM="dumb"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wq/dwn8hs0x7njbzty9f68y61700000gn/T/go-build830555530=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
uname -v: Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64
ProductName:    Mac OS X
ProductVersion: 10.11.6
BuildVersion:   15G1004
lldb --version: lldb-360.1.50
gdb --version: GNU gdb (GDB) 7.11.1