Fix extended service number calculation

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=219597894

library/core/src/main/java/com/google/android/exoplayer2/text/cea/Cea708Decoder.java

@@ -272,7 +272,10 @@ public final class Cea708Decoder extends CeaDecoder {
     if (serviceNumber == 7) {
     if (serviceNumber == 7) {
       // extended service numbers
       // extended service numbers
       serviceBlockPacket.skipBits(2);
       serviceBlockPacket.skipBits(2);
-      serviceNumber += serviceBlockPacket.readBits(6);
+      serviceNumber = serviceBlockPacket.readBits(6);
+      if (serviceNumber < 7) {
+        Log.w(TAG, "Invalid extended service number: " + serviceNumber);
+      }
     }
     }
 
 
     // Ignore packets in which blockSize is 0
     // Ignore packets in which blockSize is 0