Code Review Feedback - LendingPoolCore contract

[redlarva]

Validating Reserve, Removing fallback Method, and Sanitizing tokenFallback

• Validate Reserve for getReserveAvailableLiquidity Params:
  getReserveAvailableLiquidity method doesn't validate the reserve parameter. To mitigate this risk, we add a validation check to ensure that the reserve parameter is a valid address before proceeding with the function.

• Remove Fallback Method to receive ICX:
  In our current implementation, we have a fallback method that can receive ICX payments. As LPC does not need to receive any ICX we can remove this method.

• Sanitize tokenFallback to receive only from valid Tokens:
  Our tokenFallback method should only accept tokens from valid sources. We should add a check to ensure that the tokens being sent are from a trusted source.