scheduler: use calloc instead of malloc (#7962)

This is fixing a use-after-free if multiple schedulers are used in a
single run.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=60089

Signed-off-by: David Korczynski <[email protected]>

src/flb_scheduler.c

@@ -523,7 +523,7 @@ struct flb_sched *flb_sched_create(struct flb_config *config,
     struct flb_sched *sched;
     struct flb_sched_timer *timer;
 
-    sched = flb_malloc(sizeof(struct flb_sched));
+    sched = flb_calloc(1, sizeof(struct flb_sched));
     if (!sched) {
         flb_errno();
         return NULL;