Add check for null config in startIpClient.

Protect the null config when current network configuration can not be
retrived.

Bug: 219394598
Test: atest ClientModeImplTest
Change-Id: I27882330164836e0db25eb25fce482ce3774848a
diff --git a/service/java/com/android/server/wifi/ClientModeImpl.java b/service/java/com/android/server/wifi/ClientModeImpl.java
index 11906f4..815d369 100644
--- a/service/java/com/android/server/wifi/ClientModeImpl.java
+++ b/service/java/com/android/server/wifi/ClientModeImpl.java
@@ -6317,7 +6317,7 @@
     }
 
     private boolean startIpClient(WifiConfiguration config, boolean isFilsConnection) {
-        if (mIpClient == null) {
+        if (mIpClient == null || config == null) {
             return false;
         }