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Describe the bug
The assertion assert(ip6_hdr); in get_layer4_v6() at common/get.c is reachable when the user uses tcprewrite to open a crafted pcap file.

To Reproduce
Steps to reproduce the behavior:

1. export CC=clang && export CFLAGS="-fsanitize=address -g"
2. ./autogen.sh && ./configure --disable-shared --disable-local-libopts && make clean && make -j8
3. src/tcprewrite -o /dev/null -i POC
   output:

Warning: tcprewrite/crash.0 was captured using a snaplen of 96 bytes.  This may mean you have truncated packets.
tcprewrite: get.c:599: void *get_layer4_v6(const ipv6_hdr_t *, const int): Assertion `ip6_hdr' failed.
Aborted

Expected behavior
Program reports assertion failure and is terminated.

Screenshots

System (please complete the following information):

• OS: Ubuntu
• OS version : can be reproduced in 18.04/20.04
• Clang version : clang-12.0.1 (release/12.x)
• Tcpreplay Version : latest commit 09f0774

Credit
Han Zheng
NCNIPC of China
Hexhive