net: sockets: tls: Fix iov_len comparison in sendmsg()

vec->iov_len is of type size_t, so the comparison was always true.
Additionally, doing the memcpy() when iov_len was 0 did not really make
sense, so do it only when the actual length is larger than 0.

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