RunTx() and runCheckTxConcurrently() should contain existing handler msgs check

[dalmirel]

The following issue was reported during the Informal Systems audit of dYdX cosmos-sdk fork codebase at commit hash: f6e7e7a.

Classification of the issue:

Type	Severity	Impact	Exploitability
Implementation	Low	Medium	Low

Involved artifacts:

• cosmos-sdk/baseapp/baseapp.go

Description
A thorough analysis of Cosmos SDK’s RunTx() for modes: runTxModeCheck, runTxModeReCheck
and newly added dYdX Cosmos SDK's fork runCheckTxConcurrently() functions is performed in order to confirm that all necessary checks are being executed prior to adding the transaction to the mempool.

It has been noticed that RunMsgs() is missing a check whether the application's msgServiceRouter contains a handler for each of the messages contained in the tx, which is expected to be part of this function by the Cosmos SDK documentation.

Problem Scenarios
Without the check listed above, there is a possibility of invalid (non-executable) transactions from the mempool reaching to the proposed blocks and taking slots that could be filled with valid, executable transactions.

Recommendation
runTx() and runCheckTxConcurrently() should contain the following check:

for i, msg := range msgs {
		
  handler := app.msgServiceRouter.Handler(msg)
  if handler == nil {
		err = sdkerrors.Wrapf(sdkerrors.ErrUnknownRequest, "can't route message %+v", msg)
  }
}

The check could be placed above the critical section part here, which would be inline with the solution Cosmos SDK team applied here.