core/mbox: fix race condition

The mbox code contains a race condition in `mbox_put()`: When it
waits for a slot in the queue to become available, it is woken up with
IRQs enabled. It disables IRQs again as first thing, but by then
another thread may already have preempted the running thread and filled
the queue back up. In this case, a message in the queue would be
silently overwritten.

(cherry picked from commit 42b9334)

core/mbox.c

@@ -73,7 +73,7 @@ int _mbox_put(mbox_t *mbox, msg_t *msg, int blocking)
         return 1;
     }
     else {
-        if (cib_full(&mbox->cib)) {
+        while (cib_full(&mbox->cib)) {
             if (blocking) {
                 _wait(&mbox->writers, irqstate);
                 irqstate = irq_disable();