[automerge] fmq_fuzzer: Exit blocking threads when no more work 2p: b1f22fd744

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/libfmq/+/17453953

Bug: 218521670
Change-Id: Ic887ec8d76c322e238234ec9c67e2cc4fa00ef0a