fix vuln demonstrated by gnolang#583

Original fix at cosmos/iavl#582, is simply:

```
if len(pin.Left) > 0 && len(pin.Right) > 0 {
	return nil, errors.New("both left and right child hashes are set")
}
```
Our iavl functions however don't return errors. Proposing to use
`panic()` instead, as it does in other parts of this file.

More about this 2022-10-07 vuln:

https://medium.com/@Beosin_com/how-did-the-bnb-chain-exploiter-pass-iavl-proof-verification-an-in-depth-analysis-by-beosin-c925b77bc13e
https://twitter.com/buchmanster/status/1578879225574350848

pkgs/iavl/proof.go

@@ -22,6 +22,7 @@ var (
 
 //----------------------------------------
 
+// Contract: Left and Right can never both be set. Will result in a empty `[]` roothash
 type proofInnerNode struct {
 	Height  int8   `json:"height"`
 	Size    int64  `json:"size"`
@@ -62,6 +63,10 @@ func (pin proofInnerNode) Hash(childHash []byte) []byte {
 		err = amino.EncodeVarint(buf, pin.Version)
 	}
 
+	if len(pin.Left) > 0 && len(pin.Right) > 0 {
+		panic(fmt.Sprintf("both left and right child hashes are set"))
+	}
+
 	if len(pin.Left) == 0 {
 		if err == nil {
 			err = amino.EncodeByteSlice(buf, childHash)