Integer overflow isn't being detected in BECToken.sol

Description

Bectoken's integer overflow issue isn't being detected

How to Reproduce

console
$ myth -x BECToken.sol
No Issues being detected
...
$

Expected behavior

Integer overflow should be detected

Environment

• Mythril version: develop
• Solidity compiler and version: 0.5.0
• Python version: 3.6.3
• OS and Version: Ubuntu 17.10

该提问来源于开源项目：ConsenSys/mythril