netfilter: nf_tables: Fix dereference of null pointer flow

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_tables: Fix dereference of null pointer flow

In the case where chain->flags & NFT_CHAIN_HW_OFFLOAD is false then

nft_flow_rule_create is not called and flow is NULL. The subsequent

error handling execution via label err_destroy_flow_rule will lead

to a null pointer dereference on flow when calling nft_flow_rule_destroy.

Since the error path to err_destroy_flow_rule has to cater for null

and non-null flows, only call nft_flow_rule_destroy if flow is non-null

to fix this issue.

Addresses-Coverity: ("Explicity null dereference")