MPEG4Extractor: Check mLastTrack before parsing btrt box.

Bug: 31449945
Change-Id: I3380d494a57bb81f1a7455c64c3ab78da12c382d
diff --git a/media/libstagefright/MPEG4Extractor.cpp b/media/libstagefright/MPEG4Extractor.cpp
index 6a67fcf..77e7968 100644
--- a/media/libstagefright/MPEG4Extractor.cpp
+++ b/media/libstagefright/MPEG4Extractor.cpp
@@ -1767,6 +1767,9 @@
         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)) {