The test seemingly ensures that this code block is hit only once.
However, if we enable later activation of MMEs, it will be hit a second
(and third, ...) time. The only thing in guards against, though, is an
INIT message to the L1L2 thread, when register_enb_pending == 0, which
should only be sent once.  Therefore, functionality won't be changed if
register_enb_pending becomes negative, and the assert can be removed.