Fix divide by zero in fuzzing

[amajoke]

Some fuzzer mutations do divide by zero when l4len equals to 1.

Note that the patch affects on fuzzer ability to fuzz packets where l4len equals to 1.

Tested on:
OS: Ubuntu 18.04 x64

./tcprewrite --version
tcprewrite version: 4.3.2 (build git:v4.3.2-5-g11507365)
Copyright 2013-2018 by Fred Klassen <tcpreplay at appneta dot com> - AppNeta
Copyright 2000-2012 by Aaron Turner <aturner at synfin dot net>
The entire Tcpreplay Suite is licensed under the GPLv3
Cache file supported: 04
Not compiled with libdnet.
Compiled against libpcap: 1.8.1
64 bit packet counters: enabled
Verbose printing via tcpdump: enabled
Fragroute engine: disabled

Minimized test case as an attachment. FPE-6a2-06c-a94.zip

To reproduce, extract the .zip and run:

: ./tcprewrite --verbose --fuzz-seed=2 --fuzz-factor=2 -i ./FPE-6a2-06c-a94 -o /dev/null
reading from file -, link-type EN10MB (Ethernet)
21:33:38.1113852818 IP 204.178.31.8 > 192.168.1.33: [|tcp]
AddressSanitizer:DEADLYSIGNAL
=================================================================
==16839==ERROR: AddressSanitizer: FPE on unknown address 0x0000004df6a2 (pc 0x0000004df6a2 bp 0x631000000817 sp 0x7ffe24ee0740 T0)
    #0 0x4df6a2 in fuzzing /tcpreplay/src/tcpedit/fuzzing.c:244:38
    #1 0x4c806c in tcpedit_packet /tcpreplay/src/tcpedit/tcpedit.c:273:18
    #2 0x4c6a94 in rewrite_packets /tcpreplay/src/tcprewrite.c:291:22
    #3 0x4c5a22 in main /tcpreplay/src/tcprewrite.c:130:9
    #4 0x7fbf61913b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #5 0x41be59 in _start (/tcpreplay/src/tcprewrite+0x41be59)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: FPE /tcpreplay/src/tcpedit/fuzzing.c:244:38 in fuzzing
==16839==ABORTING

I used AddressSanitizer for better stack trace, but divide by zero reproduces even without it.