libnftables: Drop cache in error case

libnftables: Drop cache in error case

If a transaction is rejected by the kernel (for instance due to a
semantic error), cache contents are potentially invalid. Release the
cache in that case to avoid the inconsistency.

The problem is easy to reproduce in an interactive session:

| nft> list ruleset
| table ip t {
| 	chain c {
| 	}
| }
| nft> flush ruleset; add rule ip t c accept
| Error: No such file or directory
| flush ruleset; add rule ip t c accept
|                            ^
| nft> list ruleset
| nft>

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>