net: fix NPE when reading IP configurations

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>

services/core/java/com/android/server/net/IpConfigStore.java

@@ -322,8 +322,11 @@ public class IpConfigStore {
                                     gateway = InetAddresses.parseNumericAddress(in.readUTF());
                                 }
                                 // If the destination is a default IPv4 route, use the gateway
-                                // address unless already set.
-                                if (dest.getAddress() instanceof Inet4Address
+                                // address unless already set. If there is no destination, assume
+                                // it is default route and use the gateway address in all cases.
+                                if (dest == null) {
+                                    gatewayAddress = gateway;
+                                } else if (dest.getAddress() instanceof Inet4Address
                                         && dest.getPrefixLength() == 0 && gatewayAddress == null) {
                                     gatewayAddress = gateway;
                                 } else {