Fix dns_fwdtable_addfwd() error path cleanup bug

Arаm Sаrgsyаn requested to merge aram/dns_fwdtable_addfwd-cleanup-bugfix into main

Free 'sizeof(dns_forwarder_t)' bytes of memory instead of 'sizeof(dns_sockaddr_t)' bytes, because fwd is a pointer to a 'dns_forwarder_t' type structure.

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