encoding/asn1: better error message when unmarshaling non-pointer

[phayes]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   NaCL / Playground
2. What operating system and processor architecture are you using (go env)?
   Playground
3. What did you do?
   https://play.golang.org/p/TnLoso9len
4. What did you expect to see?

1.2.840.113549.1.9.16.1.4
[6 11 42 134 72 134 247 13 1 9 16 1 4]
1.2.840.113549.1.9.16.1.4

1. What did you see instead?

1.2.840.113549.1.9.16.1.4
[6 11 42 134 72 134 247 13 1 9 16 1 4]
panic: reflect: call of reflect.Value.Elem on slice Value

goroutine 1 [running]:
panic(0x184720, 0x10434260)
    /usr/local/go/src/runtime/panic.go:464 +0x700
reflect.Value.Elem(0x188f40, 0x10434250, 0x97, 0x10434250, 0x0, 0x0, 0x0, 0x188f40)
    /usr/local/go/src/reflect/value.go:735 +0x2a0
encoding/asn1.UnmarshalWithParams(0x10444074, 0xd, 0x40, 0x188f40, 0x10434250, 0x0, 0x0, 0x10434220, 0x0, 0x0, ...)
    /usr/local/go/src/encoding/asn1/asn1.go:989 +0xc0
encoding/asn1.Unmarshal(0x10444074, 0xd, 0x40, 0x188f40, 0x10434250, 0x10434250, 0x0, 0x0, 0x0, 0x0, ...)
    /usr/local/go/src/encoding/asn1/asn1.go:983 +0x80
main.main()
    /tmp/sandbox412831617/main.go:24 +0x480

[martisch]

not a bug in asn1.go.

in https://play.golang.org/p/TnLoso9len

_, err = asn1.Unmarshal(data, oid) 

needs to be

_, err = asn1.Unmarshal(data, &oid)

"uses the reflect package to fill in an arbitrary value pointed at by val"
The pointed at is important.

But maybe it is warranted to add a check and write out a better error.

[phayes]

Gah! That's embarrassing! Let me change the title to reflect this.