realtime_zsl: Avoid reading pending_request after erasure

The entry erased by:
  pending_frame_number_to_requests_.erase(result->frame_number);
is actually the `pending_request`. So after the erasure it must not be
accessed again.

This commit fixes a use-after-free bug introduced by commit 31fa05e.

Fix: 279845929
Test: CTS
Change-Id: I91197513377b5e3f7972c3f521cfbd94614ae36f