NULL pointer dereference in DXF parser, HATCH code 93 #1468

Steps to reproduce or sample file

1. Unzip and load the attached proof of concept file in LibreCAD 2.2.0-rc3

Cause

The std::shared_ptr DRW_Hatch::loop is written to when loading a HATCH entity with code 93. If this occurs before a code 92, the pointer is still NULL, leading to a crash.

Impact

Denial of service.

Proposed Mitigation

Ensure that DRW_Hatch::loop is not NULL before dereferencing at drw_entities.cpp:1808

Operating System and LibreCAD version info

Version: 2.2.0-rc3
Compiler: GNU GCC 7.3.0
Compiled on: Nov 29 2021
Qt Version: 5.12.4
Boost Version: 1.65.1
System: Windows 10 (10.0)