pppoe: fix memcpy out of bounds with gcc-11 on arm

In function ‘memcpy’,
    inlined from ‘clib_memcpy_fast’ at /home/vpp/src/vppinfra/string.h:86:10,
    inlined from ‘memcpy_s_inline’ at /home/vpp/src/vppinfra/string.h:157:7,
    inlined from ‘vnet_pppoe_add_del_session’ at /home/vpp/src/plugins/pppoe/pppoe.c:356:7:
 error: ‘__builtin_memcpy’ offset [0, 5] is out of the bounds [0, 0] [-Werror=array-bounds]
   34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Cc1: all warnings being treated as errors

Hardware address is zero length vector for PPP, use vec_len instead.

Type: fix
Fixes: 62f9cdd82c52 ("Add PPPoE Plugin")
Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Change-Id: If9fb409cfbbac77c15559d103987f0130bf30255