Fix crashed multicast example application
Fix crashed multicast example application

Fixes: #146 - mcasttest thread terminate once startup

Root cause: ofp_multicast_thread use automatic variable
odph_linux_pthread_t as odph_linux_pthread_create argument. This
argument becomes invalid when mcasttest thread use it which cause the
segment fault.
Fix the issue by making it a static variable

Signed-off-by: Dapeng Liu <2a4cd463150ae38e66022a116d1b4d46734be600@linaro.org>
Reviewed-by: Sorin Vultureanu <8013ba55f8675034bc2ab0d6c3a1c9650437ca36@enea.com>

OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp,TolikH/ofp,TolikH/ofp,TolikH/ofp