ConfigEditFragment: Add extremely basic validation

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/ConfigEditFragment.java

@@ -73,7 +73,11 @@ public class ConfigEditFragment extends BaseConfigFragment {
     }
 
     private void saveConfig() {
-        // FIXME: validate input
+        final String errorMessage = localConfig.validate();
+        if (errorMessage != null) {
+            Toast.makeText(getActivity(), errorMessage, Toast.LENGTH_SHORT).show();
+            return;
+        }
         try {
             if (getCurrentConfig() != null)
                 VpnService.getInstance().update(getCurrentConfig().getName(), localConfig);