mp4: fix null deref

This used one too many derefs, and then due to the earlier write of null
near the beginning of the function (making a null write here redundant)
there was thus a null-deref.

Regression from 0e9db4f7e6.

modules/demux/mp4/attachments.c

@@ -371,7 +371,6 @@ size_t MP4_GetAttachments( const MP4_Box_t *p_root, input_attachment_t ***ppp_at
     if ( i_count == 0 )
     {
         free( pp_attach );
-        **ppp_attach = NULL;
         return 0;
     }