net: context: Fix ambigous pointer check in net_context_connect()

Coverity reported, that laddr pointer used in net_context_connect()
could be passed as NULL to net_tcp_connect() where it could be
dereferenced. This is because the actual setting of laddr to a valid
address structure was only done after
net_sin/sin6_ptr(&context->local)->sin/sin6_addr verification.

In practice though, the aforementioned pointer verification would always
pass, as the bind_default() guarantee that the context->local address is
set to an unspecified address (if it hasn't been set earlier).

Therefore refactor the code a bit: replace the pointer verification
with NET_ASSERT - only to assure that we can catch regression in case
for any reason the behavior of bind_default() changes. This should also
ensure that Coverity no longer reports that laddr is NULL when reaching
net_tcp_connect().

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