Fix a few potential crashes

Change-Id: Ib7bdfc1d5fff928570e4fec3f3d4e175f9656dd3
diff --git a/media/libmediaplayerservice/nuplayer/GenericSource.cpp b/media/libmediaplayerservice/nuplayer/GenericSource.cpp
index c5b8301..0c774bd 100644
--- a/media/libmediaplayerservice/nuplayer/GenericSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/GenericSource.cpp
@@ -216,6 +216,10 @@
         }
 
         sp<MetaData> meta = extractor->getTrackMetaData(i);
+        if (meta == NULL) {
+            ALOGE("no metadata for track %zu", i);
+            return UNKNOWN_ERROR;
+        }
 
         const char *mime;
         CHECK(meta->findCString(kKeyMIMEType, &mime));
@@ -1125,6 +1129,10 @@
 
     sp<AMessage> format = new AMessage();
     sp<MetaData> meta = mSources.itemAt(trackIndex)->getFormat();
+    if (meta == NULL) {
+        ALOGE("no metadata for track %zu", trackIndex);
+        return NULL;
+    }
 
     const char *mime;
     CHECK(meta->findCString(kKeyMIMEType, &mime));