Avoid NPE when missing network stats.

Bug: 5750339
Change-Id: I4b739dbd076b270a5a2b2e2a1e0f6e445d8c124e

Author: jsharkey

services/java/com/android/server/net/NetworkStatsService.java

@@ -1060,8 +1060,10 @@ private void removeUidLocked(int uid) {
         }
 
         // clear UID from current stats snapshot
-        mLastPollUidSnapshot = mLastPollUidSnapshot.withoutUid(uid);
-        mLastPollNetworkXtSnapshot = computeNetworkXtSnapshotFromUid(mLastPollUidSnapshot);
+        if (mLastPollUidSnapshot != null) {
+            mLastPollUidSnapshot = mLastPollUidSnapshot.withoutUid(uid);
+            mLastPollNetworkXtSnapshot = computeNetworkXtSnapshotFromUid(mLastPollUidSnapshot);
+        }
 
         // clear kernel stats associated with UID
         resetKernelUidStats(uid);