Shore up dividing by zero protections.

We're already trying to protect against zero elsewhere in this file,
but we've seen evidence of it still happening, so add last-ditch
protections.

Test: builds, boots
Bug: 73060623
Change-Id: I0549ed08f906521a8dbd9ff6162f80fa9b388434

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

@@ -187,6 +187,7 @@ public class NetworkStatsCollection implements FileRotator.Reader {
      */
     @VisibleForTesting
     public static long multiplySafe(long value, long num, long den) {
+        if (den == 0) den = 1;
         long x = value;
         long y = num;