modify_senders: re-introduce check for modification-in-progress

Since I7b6e015fa5fc6dc4492c44d9c81e38f3ef531f51 the modify_senders
operation is protected against concurrent modifications of the sender
list from threads on a different CPU and against vanishing the target
thread. But we still need to detect if the modify_senders operation is
currently in progress by a thread running on the same CPU.

See ticket #CD-1131.
See ticket #CD-1159.

Change-Id: I10daaa00a198e652ac0b4bd7a1e773e6ceb5be52

src/kern/thread_object.cpp

@@ -316,7 +316,8 @@ Thread_object::sys_modify_senders(L4_msg_tag tag, Utcb const *in, Utcb * /*out*/
   if (current_cpu() != home_cpu())
     return commit_result(-L4_err::EInval);
 
-  assert(!sender_list()->cursor());
+  if (sender_list()->cursor())
+    return commit_result(-L4_err::EBusy);
 
   if (0)
     printf("MODIFY ID (%08lx:%08lx->%08lx:%08lx\n",