aio-win32: fix uninitialized use of have_select_revents

Always initialize it with the return value of aio_prepare.
Reported-by: NTeLeMan <geleman@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

aio-win32.c

@@ -283,9 +283,9 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int count;
     int timeout;
 
-    if (aio_prepare(ctx)) {
+    have_select_revents = aio_prepare(ctx);
+    if (have_select_revents) {
         blocking = false;
-        have_select_revents = true;
     }
 
     was_dispatching = ctx->dispatching;