net: tcp: Verify if IPv4/IPv6 is enabled before processing addr family

TCP processed IPv4/IPv6 packets w/o verifying first if IPv4/IPv6 is
enabled in the system. This could lead to problems especially for IPv6,
where in case it's disabled the sockaddr structure is not large enough
to accomodate IPv6 address, leading to possible out-of-bound access on
the sockaddr structure.

Fix this by adding appropriate checks where applicable.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>