MPEG4Extractor: Check mLastTrack before parsing btrt box.

Bug: 31449945
Change-Id: I3380d494a57bb81f1a7455c64c3ab78da12c382d
(cherry picked from commit fd9cc97d4dfe2a2fbce2c0f1704d7a27ce7cbc44)
diff --git a/media/libstagefright/MPEG4Extractor.cpp b/media/libstagefright/MPEG4Extractor.cpp
index e7fe0d1..76db3fb 100644
--- a/media/libstagefright/MPEG4Extractor.cpp
+++ b/media/libstagefright/MPEG4Extractor.cpp
@@ -1778,6 +1778,9 @@
         case FOURCC('b', 't', 'r', 't'):
         {
             *offset += chunk_size;
+            if (mLastTrack == NULL) {
+                return ERROR_MALFORMED;
+            }
 
             uint8_t buffer[12];
             if (chunk_data_size != sizeof(buffer)) {