[libFuzzer] processes stop working using `-fork`

Processes will stop working after while when running a fuzzer with `-fork`. The processes all still exists but they are not doing any work (i.e. no CPU usage). This deteriorates to the point where no process is doing any more work (in my case the last result was returned after 3275 seconds).

This does not appear to be related to any crashes (the detected amount is far less than the overall processes used) or timeouts (none detected and they are set to 5 seconds).

`ps axww`
```
66725 pts/1 S+ 0:00 sh -c -- ./oss-fuzz-client -only_ascii=1 -timeout=5 -ignore_crashes=1 -ignore_timeouts=1 -use_value_profile=1 -reload=0 -print_final_stats=1 -print_funcs=0 -max_total_time=186 -stop_file=/tmp/libFuzzerTemp.FuzzWithFork60004.dir/STOP -seed_inputs=@/tmp/libFuzzerTemp.FuzzWithFork60004.dir/186.seeds /tmp/libFuzzerTemp.FuzzWithFork60004.dir/C186 -features_dir=/tmp/libFuzzerTemp.FuzzWithFork60004.dir/F186 >/tmp/libFuzzerTemp.FuzzWithFork60004.dir/186.log 2>&1
66726 pts/1 Sl+ 0:54 ./oss-fuzz-client -only_ascii=1 -timeout=5 -ignore_crashes=1 -ignore_timeouts=1 -use_value_profile=1 -reload=0 -print_final_stats=1 -print_funcs=0 -max_total_time=186 -stop_file=/tmp/libFuzzerTemp.FuzzWithFork60004.dir/STOP -seed_inputs=@/tmp/libFuzzerTemp.FuzzWithFork60004.dir/186.seeds /tmp/libFuzzerTemp.FuzzWithFork60004.dir/C186 -features_dir=/tmp/libFuzzerTemp.FuzzWithFork60004.dir/F186
```

```
strace: Process 66725 attached
wait4(-1
```

```
strace: Process 66726 attached
futex(0x7f6e7e608ac0, FUTEX_WAIT_PRIVATE, 2, NULL
```

Start by reproducing the stall with libFuzzer's `-fork` command and inspect the waiting processes shown by `ps` and `strace`. Trace the `-fork` execution path and determine why worker processes stop making progress; done means the processes continue fuzzing and complete without becoming stuck.

Written by the indexing model from the issue text.