parser_bison: close chain scope before chain release

parser_bison: close chain scope before chain release

cmd_alloc() will free the chain, so we must close the scope opened
in chain_block_alloc beforehand.

The included test file will cause a use-after-free because nft attempts
to search for an identifier in a scope that has been freed:

AddressSanitizer: heap-use-after-free on address 0x618000000368 at pc 0x7f1cbc0e6959 bp 0x7ffd3ccb7850 sp 0x7ffd3ccb7840
    #0 0x7f1cbc0e6958 in symbol_lookup src/rule.c:629
    #1 0x7f1cbc0e66a1 in symbol_get src/rule.c:588
    #2 0x7f1cbc120d67 in nft_parse src/parser_bison.y:4325

Fixes: a66b5ad9540d ("src: allow for updating devices on existing netdev chain")
Signed-off-by: Florian Westphal <fw@strlen.de>