chore: fix deref of null

Return value of a function 'space_cache_find' is dereferenced at
trigger.c:427 without checking for NULL, but it is usually checked for
this function (32/34).

certification_patches/svace_patches/tarantool-sys_trigger.patch

+diff --git a/src/box/sql/trigger.c b/src/box/sql/trigger.c
+index 9b5e74320..993c9ae97 100644
+--- a/src/box/sql/trigger.c
++++ b/src/box/sql/trigger.c
+@@ -422,6 +422,8 @@ struct sql_trigger *
+ space_trigger_list(uint32_t space_id)
+ {
+ 	struct space *space = space_cache_find(space_id);
++	if (space == NULL) // assert
++		panic("rc == NULL in %s:%d", __FILE__, __LINE__);
+ 	assert(space != NULL);
+ 	assert(space->def != NULL);
+ 	return space->sql_triggers;