finally fixed CID 53957

It seemed that this issue had more sub-issues, that's why it was marked
as resolved while it wasn't from Coverity POV.

ldgm/src/ldgm-session-cpu.cpp

@@ -406,6 +406,7 @@ LDGM_session_cpu::decode_frame ( char* received, int buf_size, int* frame_size,
 
     int undecoded = 0;
     it = graph.nodes.find(0);
+    assert(it != graph.nodes.end());
     while ( it != graph.nodes.find(param_k) ) {
         if (!(*it).second.isDone())
             undecoded++;