aio-win32: avoid out-of-bounds access to the events array

If ret is WAIT_TIMEOUT and there was an event returned by select(),
we can write to a location after the end of the array.  But in
that case we can retry the WaitForMultipleObjects call with the
same set of events, so just move the event[ret - WAIT_OBJECT_0]
assignment inside the existin conditional.

Reported-by: TeLeMan <geleman@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: TeLeMan <geleman@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

aio-win32.c

@@ -335,6 +335,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
         event = NULL;
         if ((DWORD) (ret - WAIT_OBJECT_0) < count) {
             event = events[ret - WAIT_OBJECT_0];
+            events[ret - WAIT_OBJECT_0] = events[--count];
         } else if (!have_select_revents) {
             break;
         }
@@ -343,9 +344,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
         blocking = false;
 
         progress |= aio_dispatch_handlers(ctx, event);
-
-        /* Try again, but only call each handler once.  */
-        events[ret - WAIT_OBJECT_0] = events[--count];
     }
 
     progress |= timerlistgroup_run_timers(&ctx->tlg);