net: context: Reset peer address on connect with AF_UNSPEC

As per POSIX, for non-connection-mode sockets the peer address should be
reset if AF_UNSPEC family type is provided to the connect() call:

  "If the sa_family member of address is AF_UNSPEC, the socket's peer
   address shall be reset.""

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