#1526 break observer with relationship

[codehz]

Describe the bug
When updated to latest snapshot version, I found my program is broken, I'm not sure if this is a correct usage, but it worked in previous versions

To Reproduce
mininal repo:

#include "flecs.h"

static void TestObserver(ecs_iter_t *it) {}

int main() {
  ecs_world_t *ecs = ecs_init();

  ECS_TAG(ecs, Tag);
  ECS_TAG(ecs, Rel);

  ECS_OBSERVER(ecs, TestObserver, EcsMonitor, Tag, Tag($o), (Rel, $o));

  ecs_entity_t a = ecs_new(ecs);
  ecs_entity_t b = ecs_new(ecs);
  ecs_defer_begin(ecs);
  ecs_add(ecs, a, Tag);
  ecs_add(ecs, b, Tag);
  ecs_add_pair(ecs, a, Rel, b);
  ecs_defer_end(ecs);

  ecs_fini(ecs);
}

result:

fatal: flecs.c: 15175: assert: observer_src != 0 INTERNAL_ERROR
1   test                                0x0000000102631238 flecs_log_msg + 1676
2   test                                0x0000000102656dd8 ecs_printv_ + 212
3   test                                0x000000010260a68c ecs_log_ + 100
4   test                                0x00000001025ff848 ecs_assert_log_ + 264
5   test                                0x000000010262cb7c flecs_uni_observer_invoke + 928
6   test                                0x000000010262bd18 flecs_observers_invoke + 328
7   test                                0x0000000102605018 flecs_emit + 6940
8   test                                0x00000001026e67c4 flecs_notify_on_add + 532
9   test                                0x000000010261c698 flecs_cmd_batch_for_entity + 2820
10  test                                0x0000000102609858 flecs_defer_end + 1724
11  test                                0x000000010260738c ecs_defer_end + 180
12  test                                0x00000001025f6e68 main + 932
13  dyld                                0x0000000190050274 start + 2840
fish: Job 1, './test' terminated by signal SIGABRT (Abort)

Expected behavior
not crash

Additional context
reference: #1526

[SanderMertens]

Fixed!