[Code Analysis] Report by sonarcloud.io

Description

The code analysis report by https://sonarcloud.io/project/issues?id=org.openapitools%3Aopenapi-generator-project&resolved=false&types=BUG uncovers some bugs and vulnerabilities.

openapi-generator version

Latest master

Suggest a fix/enhancement

@ymohdriz is very kind to offer help to fix the bugs in the report. If anyone wants to help out (e.g. fixing the vulnerabilities), please reply to let us know.