core: msg: yield after queueing of a message if REPLY_BLOCKED

When setting the running task reply_blocked, it is implicitly removed
from the runqueue. But if queueing of a msg is actually successful, the
thread exits msg_send without yielding, continuing to run even if it's
not supposed to.

Nice example of why multiple function exit points lead to weird
errors...

core/msg.c

@@ -78,6 +78,9 @@ int msg_send(msg_t *m, unsigned int target_pid, bool block)
         if (target->msg_array && queue_msg(target, m)) {
             DEBUG("msg_send() %s:%i: Target %i has a msg_queue. Queueing message.\n", __FILE__, __LINE__, target_pid);
             eINT();
+            if (active_thread->status == STATUS_REPLY_BLOCKED) {
+                thread_yield();
+            }
             return 1;
         }