rpc/jsonrpc: Unmarshal RPCRequest correctly (bp #6191) (#6193)

* rpc/jsonrpc: Unmarshal RPCRequest correctly (#6191)

i.e. without double pointer. With double pointer, it was possible to
submit `null` value, which will crash the server.

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x189ddc0]

goroutine 1 [running]:
github.com/tendermint/tendermint/rpc/jsonrpc/types.(*RPCRequest).UnmarshalJSON(0xc0000147e0, 0xc00029f201, 0x4, 0x1ff, 0x883baa0, 0xc0000147e0)
        /Users/anton/go/src/github.com/tendermint/tendermint/rpc/jsonrpc/types/types.go:70 +0x100
encoding/json.(*decodeState).literalStore(0xc000216bb0, 0xc00029f201, 0x4, 0x1ff, 0x1998800, 0xc0000147e0, 0x199, 0xc000231700, 0x10e0a5e, 0x197)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:860 +0x30ce
encoding/json.(*decodeState).value(0xc000216bb0, 0x1998800, 0xc0000147e0, 0x199, 0x1998800, 0xc0000147e0)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:384 +0x40c
encoding/json.(*decodeState).array(0xc000216bb0, 0x18df040, 0xc0001be540, 0x16, 0xc000216bd8, 0x10e405b)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:558 +0x365
encoding/json.(*decodeState).value(0xc000216bb0, 0x18df040, 0xc0001be540, 0x16, 0x16, 0x6e)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:360 +0x22f
encoding/json.(*decodeState).unmarshal(0xc000216bb0, 0x18df040, 0xc0001be540, 0xc000216bd8, 0x0)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:180 +0x2c9
encoding/json.Unmarshal(0xc00029f200, 0x6, 0x200, 0x18df040, 0xc0001be540, 0x0, 0x0)
        /usr/local/Cellar/go/1.16/libexec/src/encoding/json/decode.go:107 +0x15d
```

(cherry picked from commit fe4e97afe0)

# Conflicts:
#	CHANGELOG_PENDING.md

* fix conflict

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>

CHANGELOG_PENDING.md

@@ -24,3 +24,4 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 
 ### BUG FIXES
 
+- [rpc/jsonrpc/server] \#6191 Correctly unmarshal `RPCRequest` when data is `null` (@melekes)

rpc/jsonrpc/types/types.go

@@ -57,27 +57,31 @@ type RPCRequest struct {
 
 // UnmarshalJSON custom JSON unmarshalling due to jsonrpcid being string or int
 func (req *RPCRequest) UnmarshalJSON(data []byte) error {
-	unsafeReq := &struct {
+	unsafeReq := struct {
 		JSONRPC string          `json:"jsonrpc"`
 		ID      interface{}     `json:"id,omitempty"`
 		Method  string          `json:"method"`
 		Params  json.RawMessage `json:"params"` // must be map[string]interface{} or []interface{}
 	}{}
+
 	err := json.Unmarshal(data, &unsafeReq)
 	if err != nil {
 		return err
 	}
+
+	if unsafeReq.ID == nil { // notification
+		return nil
+	}
+
 	req.JSONRPC = unsafeReq.JSONRPC
 	req.Method = unsafeReq.Method
 	req.Params = unsafeReq.Params
-	if unsafeReq.ID == nil {
-		return nil
-	}
 	id, err := idFromInterface(unsafeReq.ID)
 	if err != nil {
 		return err
 	}
 	req.ID = id
+
 	return nil
 }