keep tx3g{Buffer,Size,Filled} in sync

wasn't clearing size/filled when we cleared buffer.

Bug: 151159638
Test: poc, but it didn't fail either before or after the patch

media/extractors/mp4/MPEG4Extractor.cpp

@@ -1075,6 +1075,8 @@ status_t MPEG4Extractor::parseChunk(off64_t *offset, int depth) {
                     // drop it now to reduce our footprint
                     free(mLastTrack->mTx3gBuffer);
                     mLastTrack->mTx3gBuffer = NULL;
+                    mLastTrack->mTx3gFilled = 0;
+                    mLastTrack->mTx3gSize = 0;
                 }
 
                 const char *mime;
@@ -2787,6 +2789,10 @@ status_t MPEG4Extractor::parseChunk(off64_t *offset, int depth) {
             // if those apps are compensating for it, we'd break them with such a change
             //
 
+            if (mLastTrack->mTx3gBuffer == NULL) {
+                mLastTrack->mTx3gSize = 0;
+                mLastTrack->mTx3gFilled = 0;
+            }
             if (mLastTrack->mTx3gSize - mLastTrack->mTx3gFilled < chunk_size) {
                 size_t growth = kTx3gGrowth;
                 if (growth < chunk_size) {