Decode panics on nil interface with pre-populated typed pointer (upstream #304)

[xe-nvdk]

Upstream: vmihailenco#304

When decoding a struct where an interface{} field was encoded as nil, but the decode target has that field pre-populated with a typed pointer (e.g., Data: &Recipe{}), the decoder panics with reflect.Value.Set using unaddressable value.

This is the canonical issue for a family of related panics:

• upstream use msgpack.rawmessage to decode panic reflect: reflect.Value.Set using unaddressable value vmihailenco/msgpack#310 (RawMessage decode panic, has PR fixed bugs vmihailenco/msgpack#309 with fix)
• upstream panic: reflect: reflect.Value.SetString using unaddressable value vmihailenco/msgpack#328 (SetString panic, minimal info)
• upstream panic: reflect: reflect.Value.SetString using unaddressable value vmihailenco/msgpack#336 (SetString panic with variadic marshal)
• upstream panic: reflect: reflect.Value.SetString using unaddressable value vmihailenco/msgpack#339 (SetString panic with empty struct pointer)

Root cause is in ptrValueDecoder / decodeInterfaceValue — nil-handling path does not check v.CanSet().

Proposed fix (from upstream): Check for nil code in decodeInterfaceValue and use reflect.Zero to safely clear the value.