parser_bison: fix memory leak when parsing flowtable hook declaration

parser_bison: fix memory leak when parsing flowtable hook declaration

When the hook location is invalid we error out but we do leak both
the priority expression and the flowtable name.  Example:

valgrind --leak-check=full nft -f flowtable-parser-err-memleak
[..] Error: unknown chain hook
hook enoent priority filter + 10
     ^^^^^^
[..]
2 bytes in 1 blocks are definitely lost in loss record 1 of 3
   at: malloc (vg_replace_malloc.c:446)
   by: strdup (in libc.so.6)
   by: xstrdup (in libnftables.so.1.1.0)
   by: nft_lex (in libnftables.so.1.1.0)
   by: nft_parse (in libnftables.so.1.1.0)
   by: __nft_run_cmd_from_filename (in libnftables.so.1.1.0)
   by: nft_run_cmd_from_filename (in libnftables.so.1.1.0)

First two reports are due to the priority expression: this needs to call
expr_free().  Third report is due to the flowtable name, the destructor
was missing so add one.

After fix:
All heap blocks were freed -- no leaks are possible

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>