Fix FD crash when the SD disk is full introduced with SDPacketCheck feature

Fix FD crash when the SD disk is full introduced with SDPacketCheck feature

Description:
------------

The FD crash when the SD send back a message to the FD (for example when the disk is full)
This issue was introduced in 12.6.2 with SDPacketCheck feature

Workaround:
-----------

Set SDPacketCheck to some value in the bacula-fd.conf

Details:
--------

bnet_poll_manager cannot handle error msg from SD

- FD get a "3999 Failed append\n" in the hearbeat while jcr->sd_packet_mgr
  was already set to NULL