net: context: Verify that laddr was set before use in connect

In previous patch fixing this issue, I've missed the fact that offloaded
drivers would not set the context->local address, which resulted in a
regression, where the previously introduced assert would hit in
offloaded cases. Not setting laddr is not a problem in case of
offloading, as it's only used in net_tcp_connect() which would not be
reached in this case.

Therefore I propose to remove previous patch to get rid of regression.
As an alternative fix, verify the laddr just before use, so that it is
only checked when native net stack is in use.

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