Fix maybe uninitialized warning in src/bm_sim/pcap_file.cpp
Fix maybe uninitialized warning in src/bm_sim/pcap_file.cpp

See issue #480

hanw/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model