[PX] Smart Contract Audit

We use Slither, the Solidity source analyzer for preliminary [PX] smart contract code audit.

Note: Slither is a Solidity static analysis framework written in Python3. It runs a suite of vulnerability detectors, prints visual information about contract details, and provides an API to easily write custom analyses. Slither enables developers to find vulnerabilities, enhance their code comprehension, and quickly prototype custom analyses.