This patch restores the original behaviour of Cooja when
the transmitted packet is correct (which is true in a vast majority
of cases).
In case of a wrong outgoing packet (wrong length, wrong preamble)
the transmission will end when the radio changes its state (which
should always happen after transmitting a packet).
Benchmarks with RPL (33 runs, 50 nodes, 3 hours of simulated time each)
yield the same results (PDR, delay, number of transmitted packets)
as with the unmodified Cooja.
eb4bdde9ca