Fix issues on EBML lacing block parsing

Fix EBML lacing block parsing when attempting to parse the last frame
on single-frame videos and assertion when frame_count is lower
than actual data in the file.

Change-Id: I223e30be54970ef75126c6c4ee3401cbeb92e02f

mkvparser.cpp

@@ -7141,7 +7141,8 @@ long Block::Parse(const Cluster* pCluster) {
       --frame_count;
     }
 
-    {
+    // parse last frame
+    if (frame_count > 0) {
       assert(pos <= stop);
       assert(pf < pf_end);
 
@@ -7179,7 +7180,8 @@ long Block::Parse(const Cluster* pCluster) {
       pos += f.len;
     }
 
-    assert(pos == stop);
+    if (pos != stop)
+      return E_FILE_FORMAT_INVALID;
   }
 
   return 0;  // success