Bio Cannibalize - Slave Problem

[Vaans]

Describe the bug
When Bio Cannibalize is used, many spam errors appear in the console.

To Reproduce
Steps to reproduce the behavior:

1. Use the latest update
2. Use Bio cannibalize in any level
3. See the error in the console

Expected behavior
No error messages.

System specs (please complete the following information):

• OS: Linux 3.10.0-693.2.2.el7.x86_64 - Centos 7
• Hercules Version Release v2019.10.20
• Mode: Pre-Renewal

Additional context

[Error]: --- nullpo info --------------------------------------------
[Error]: mob.c:1385: 'm_md' in function `mob_ai_sub_hard_slavemob'
[Error]: ./map-server() [0x630513]
[Error]: ./map-server() [0x4f552d]
[Error]: ./map-server() [0x4f322d]
[Error]: ./map-server() [0x4ec213]
[Error]: ./map-server() [0x4dd658]
[Error]: ./map-server() [0x4e3f66]
[Error]: ./map-server() [0x4dd1b1]
[Error]: ./map-server() [0x4f5e9d]
[Error]: ./map-server() [0x4dda14]
[Error]: ./map-server() [0x4dd4d3]
[Error]: --- end nullpo info ----------------------------------------

https://github.com/HerculesWS/Hercules/pull/2561