Merkle Proof verification failed for a valid proof.

https://github.com/ConsenSys/rb-relay/blob/4043a0bf22dbe8f896cd9b03861492adbdfa8091/contracts/MerklePatriciaProof.sol#L19

We are using `MerklePatriciaProof.sol` library in [Mosaic-contracts](https://github.com/OpenSTFoundation/mosaic-contracts). While testing, we came across a use case where verification of Merkle proof failed for valid proof.

After debugging, a scenario is observed for the merkle proof which has an extension node in between.

On[ L53,`_nibblesToTraverse`](https://github.com/ConsenSys/rb-relay/blob/4043a0bf22dbe8f896cd9b03861492adbdfa8091/contracts/MerklePatriciaProof.sol#L53) function increases pathPtr with the size of the extension node key.

`pathPtr += _nibblesToTraverse(RLP.toData(currentNodeList[0]), path, pathPtr);`

Then on [L63](https://github.com/ConsenSys/rb-relay/blob/4043a0bf22dbe8f896cd9b03861492adbdfa8091/contracts/MerklePatriciaProof.sol#L63), updated `pathPtr` is passed. Here `_nibblesToTraverse` will return `0` because the key of the extension node will not match this time. Hence `false` is returned from the function and merkle proof verification will fail.

Possible fix for the issue is to preserve `pathPtr` before updating on L53 and pass the preserved `pathPtr` on L63. Refer this [commit](https://github.com/sarvesh-ost/mosaic-contracts/commit/97714b646ada224f425463128810aae21893c58d).

Below are the parameters which were passed to the `verify` function.

*value* : 0x5fe7f977e71dba2ea1a68e21057beebb9be2ac30c6410aa38d4f3fbe41dcffd2

*encodedPath* : 0x40769c9cc6db1aff35e37923897950227d2a87c95ef0101b2a0233740f2ce00d

*rlpParentNodes* : 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

*root* : 0xa506e7e444fc636360bb593a808f41f126a8813ea71410d4515987eacc594380