runtime_events: avoid uninitialized value read

Reported by `-fsanitize=memory`:
```
> ==102752==WARNING: MemorySanitizer: use-of-uninitialized-value
>     #0 0x7f2ba7fb4ea4 in caml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:496:18
>     #1 0x7f2ba7fbc016 in caml_ml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:1207:9
>     ocaml#2 0x59ba5c in caml_interprete /var/home/edwin/git/ocaml/runtime/interp.c:1058:14
>     ocaml#3 0x5a9220 in caml_main /var/home/edwin/git/ocaml/runtime/startup_byt.c:575:9
>     ocaml#4 0x540d6b in main /var/home/edwin/git/ocaml/runtime/main.c:37:3
>     ocaml#5 0x7f2ba8120087 in __libc_start_call_main (/lib64/libc.so.6+0x2a087) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#6 0x7f2ba812014a in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x2a14a) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#7 0x441804 in _start (/var/home/edwin/git/ocaml/runtime/ocamlrun+0x441804) (BuildId: 617637580ee48eff08a2bce790e1667ad09f3b69)
>
>   Uninitialized value was stored to memory at
>     #0 0x7f2ba7fb4e9d in caml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:497:69
>     #1 0x7f2ba7fbc016 in caml_ml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:1207:9
>     ocaml#2 0x59ba5c in caml_interprete /var/home/edwin/git/ocaml/runtime/interp.c:1058:14
>     ocaml#3 0x5a9220 in caml_main /var/home/edwin/git/ocaml/runtime/startup_byt.c:575:9
>     ocaml#4 0x540d6b in main /var/home/edwin/git/ocaml/runtime/main.c:37:3
>     ocaml#5 0x7f2ba8120087 in __libc_start_call_main (/lib64/libc.so.6+0x2a087) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#6 0x7f2ba812014a in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x2a14a) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#7 0x441804 in _start (/var/home/edwin/git/ocaml/runtime/ocamlrun+0x441804) (BuildId: 617637580ee48eff08a2bce790e1667ad09f3b69)
>
>   Uninitialized value was created by an allocation of 'buf' in the stack frame
>     #0 0x7f2ba7fb3dbc in caml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:402:7
>
```

This is in fact an EV_LIFECYCLE with EV_RING_STOP, which has 0
additional data, and thus msg_length 2:
```
runtime/runtime_events.c:      EV_RUNTIME, (ev_message_type){.runtime=EV_LIFECYCLE}, EV_RING_STOP, 0,
```

Attempting to read from `buf[2]` would read uninitialized data (or
potentially beyond the end of the buffer).

Check `msg_length` before reading.

Signed-off-by: Edwin Török <[email protected]>

Author: edwintorok

otherlibs/runtime_events/runtime_events_consumer.c

@@ -493,8 +493,9 @@ caml_runtime_events_read_poll(struct caml_runtime_events_cursor *cursor,
           break;
         case EV_LIFECYCLE:
           if (cursor->lifecycle) {
+            int64_t data = msg_length > 2 ? buf[2] : 0;
             if( !cursor->lifecycle(domain_num, callback_data, buf[1],
-                                    RUNTIME_EVENTS_ITEM_ID(header), buf[2]) ) {
+                                    RUNTIME_EVENTS_ITEM_ID(header), data) ) {
                                       early_exit = 1;
                                       continue;
                                     }