Fixed 2 issues with Cea708Decoder.

The first issue occurs when we attempt to process a DtvCcPacket that hasn't been completely filled. In this case we attempted to extract data beyond the length of the packet, instead of dropping the packet as we should have. The other issue occurs when we encountered an invalid cc_data_pkt. In that case we were finalizing the entire DtvCcPacket, instead of just ignoring that particular cc_data_pkt as we should have.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146373074

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

@@ -211,7 +211,7 @@ protected void decode(SubtitleInputBuffer inputBuffer) {
       }
 
       if (!ccValid) {
-        finalizeCurrentPacket();
+        // This byte-pair isn't valid, ignore it and continue.
         continue;
       }
 
@@ -259,7 +259,8 @@ private void processCurrentPacket() {
     if (currentDtvCcPacket.currentIndex != (currentDtvCcPacket.packetSize * 2 - 1)) {
       Log.w(TAG, "DtvCcPacket ended prematurely; size is " + (currentDtvCcPacket.packetSize * 2 - 1)
           + ", but current index is " + currentDtvCcPacket.currentIndex + " (sequence number "
-          + currentDtvCcPacket.sequenceNumber + ")");
+          + currentDtvCcPacket.sequenceNumber + "); ignoring packet");
+      return;
     }
 
     serviceBlockPacket.reset(currentDtvCcPacket.packetData, currentDtvCcPacket.currentIndex);