net/colo-compare.c: Fix a crash in COLO Primary.

Because event_unhandled_count may be accessed concurrently, it needs
to be protected by taking the lock. However the assert is outside the
lock, probably causing it to read garbage and aborting Qemu erroneously.

The Bug only happens when running Qemu in COLO mode.

This Patch fixes the following bug: https://bugs.launchpad.net/qemu/+bug/1824622

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/colo-compare.c

@@ -813,9 +813,8 @@ static void colo_compare_handle_event(void *opaque)
         break;
     }
 
-    assert(event_unhandled_count > 0);
-
     qemu_mutex_lock(&event_mtx);
+    assert(event_unhandled_count > 0);
     event_unhandled_count--;
     qemu_cond_broadcast(&event_complete_cond);
     qemu_mutex_unlock(&event_mtx);