Fix PacketListField.do_copy() to work w/ strings elements

errno set by pcap_open_live(3) (typically EPERM or ENXIO) was notchecked, eventually causing a segmentation violation atpcap_get_selectable_fd():    $ python3 -c 'from scapy.all import *; conf.use_pcap = True; sendp("", iface="nonexist0")'    fish: “python3 -c 'from scapy.all impo…” terminated by signal SIGSEGV (Address boundary error)because pcap_get_selectable_fd(3) refered to NULL:    (gdb) bt    #0  pcap_get_selectable_fd (p=0x0) at ./pcap.c:1419secdev#1  0x00007f963a72781e in ffi_call_unix64 () from /lib/x86_64-linux-gnu/libffi.so.6secdev#2  0x00007f963a7271ef in ffi_call () from /lib/x86_64-linux-gnu/libffi.so.6secdev#3  0x00007f963b17cf49 in _call_function_pointer (flags=flags@entry=4353, pProc=pProc@entry=0x7f96382d1300 <pcap_get_selectable_fd>, avalues=0x7ffd96028bd0,        atypes=<optimized out>, restype=<optimized out>, resmem=resmem@entry=0x7ffd96028be0, argcount=1) at ./Modules/_ctypes/callproc.c:827secdev#4  0x00007f963b17d965 in _ctypes_callproc (pProc=pProc@entry=0x7f96382d1300 <pcap_get_selectable_fd>, argtuple=argtuple@entry=0x7f963b263608, flags=4353,        argtypes=argtypes@entry=0x7f9638360d58, restype=restype@entry=0x18d7628, checker=checker@entry=0x0) at ./Modules/_ctypes/callproc.c:1184secdev#5  0x00007f963b176401 in PyCFuncPtr_call (self=0x7f963836bcf0, inargs=<optimized out>, kwds=<optimized out>) at ./Modules/_ctypes/_ctypes.c:3969secdev#6  0x000000000043730f in ?? ()secdev#7  0x00007f96382823b8 in ?? ()secdev#8  0x0000000000000000 in ?? ()    (gdb) l    1414    1415#if !defined(_WIN32) && !defined(MSDOS)    1416int    1417pcap_get_selectable_fd(pcap_t *p)    1418{    1419return (p->selectable_fd);    1420}    1421#endif    1422    1423void    (gdb) p p    $1 = (pcap_t *) 0x0

pull from secdev

* Add support for S1G beacon to the dot11 layer. (#2) - Includes unit test parsing and confirming an S1G beacon - Includes support for the new Frame Control format for type=3,   subtype=1 (S1G beacon) - Includes changes to support addressing used in S1G beacon.All dot11 unit tests pass with this change.* More unit test changes to trigger workflows (#3)* Add support for S1G beacon to the dot11 layer. - Includes unit test parsing and confirming an S1G beacon - Includes support for the new Frame Control format for type=3,   subtype=1 (S1G beacon) - Includes changes to support addressing used in S1G beacon.All dot11 unit tests pass with this change.* Update S1G beacon parsing based on code review. (#4)* Fix review comments from@p-l-: - Use set for 'type in' checks for extension frame type 3, subtype 1   (S1G beacon).* Update to FCfield to split into three parts for type 3, subtype 1 (S1Gbeacon frame control field). This allows use of BitField for the 'bw',and FlagsField for the remaining bits.