net: Fix out-of-bounds access when registering connections

If either a remote or a local address were supplied to the
net_conn_register() function, the IP stack would proceed to copy
sizeof(struct sockaddr) bytes from the respective remote_addr
or local_addr pointers, regardless of the actual size of the storage
these pointers point to.

Use the proper size depending on the socket address family.

Coverity-ID: 173630
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>