Check mMediaProvider for null

Getting the content provider could
return null in some cases where the user
is locked, so check for that when closing.

Bug: 73656609
Test: no crash
Change-Id: I607cd2da5348520616a1c171d0d8a1afacabe592

media/java/android/mtp/MtpDatabase.java

@@ -292,7 +292,9 @@ public class MtpDatabase implements AutoCloseable {
         mCloseGuard.close();
         if (mClosed.compareAndSet(false, true)) {
             mMediaScanner.close();
-            mMediaProvider.close();
+            if (mMediaProvider != null) {
+                mMediaProvider.close();
+            }
             native_finalize();
         }
     }