MPEG4Extractor: guard against missing stbl.

Bug: 14320131
Change-Id: I33f65117c9c5365bddd96bb41cc4379a0c7ebcbd
diff --git a/media/libstagefright/MPEG4Extractor.cpp b/media/libstagefright/MPEG4Extractor.cpp
index e07b6aa..05caf75 100644
--- a/media/libstagefright/MPEG4Extractor.cpp
+++ b/media/libstagefright/MPEG4Extractor.cpp
@@ -2460,8 +2460,9 @@
         }
     }
 
-    if (!track->sampleTable->isValid()) {
+    if (track->sampleTable == NULL || !track->sampleTable->isValid()) {
         // Make sure we have all the metadata we need.
+        ALOGE("stbl atom missing/invalid.");
         return ERROR_MALFORMED;
     }