parser_bison: memleak in device parser

parser_bison: memleak in device parser

==1135425== 9 bytes in 1 blocks are definitely lost in loss record 1 of 1
==1135425==    at 0x483577F: malloc (vg_replace_malloc.c:309)
==1135425==    by 0x4BE846A: strdup (strdup.c:42)
==1135425==    by 0x48A5EDD: xstrdup (utils.c:75)
==1135425==    by 0x48C9A20: nft_lex (scanner.l:640)
==1135425==    by 0x48BC1A4: nft_parse (parser_bison.c:5682)
==1135425==    by 0x48AC336: nft_parse_bison_buffer (libnftables.c:375)
==1135425==    by 0x48AC336: nft_run_cmd_from_buffer (libnftables.c:443)
==1135425==    by 0x10A707: main (main.c:384)

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>