siri/db: add NULL check before calling siridb_tasks_dec

siri/db: add NULL check before calling siridb_tasks_dec

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/siridb-server/+bug/1987558
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1017748
Forwarded: https://github.com/SiriDB/siridb-server/pull/182
Last-Update: 2022-08-24

When built against libuv1 1.44.2, siridb_query_free may call
siridb_tasks_dec on NULL, causing a segfault. Add a NULL check on siridb
before calling siridb_tasks_dec to avoid this.
Last-Update: 2022-08-24
Gbp-Pq: Name 0002-siri-db-add-NULL-check-before-calling-siridb_tasks_d.patch