91333788a0
When the default gateway is not used in IP configurations,
for example the gateway info in ip configurations was fed with
custom gateway address
```
out.writeUTF(GATEWAY_KEY);
out.writeInt(0); // Default route.
out.writeInt(1); // Have a gateway.
out.writeUTF(staticIpConfiguration.getGateway().getHostAddress());
```
A NPE occurred.
When there is no destination, assume it is default route and use
the gateway address in all cases.
Test: manual
Change-Id: I5904efad5d277de6724f81d99e62c21ff8347caa
Signed-off-by: gary-wzl77 <gary.wang@canonical.com>