valgrind conditional jump or move depends on uninitialised value

Question

I want to fix problem reported by valgrind:

==7182== Conditional jump or move depends on uninitialised value(s)
==7182==    at 0x40EC75C: strstr (in /lib/libc-2.9.so)
==7182==    by 0x804A977: search_graph_begin (compression.c:462)
==7182==    by 0x804AB60: search_graph_end (compression.c:497)
==7182==    by 0x804AA97: search_graph_begin (compression.c:477)
==7182==    by 0x804B59A: do_g_decompress (compression.c:767)
==7182==    by 0x804996C: main (server.c:302)

my relevant part of code is:

    void search_graph_begin(char* buf, FILE *dest,int* graph_count,int extension,
  char* graphs,char* directory,int have)
    {
 char* begingraph = NULL;
 begingraph = strstr(buf,"<GRAPH>");
 if (begingraph != NULL)
 {
  if ( (int)(begingraph - buf) > 1)
  {
   printf("(int)(begingraph-buf) %d\n",(int)(begingraph-buf));
   xwrite(dest,buf,(int)(begingraph-buf));
  }
  (*graph_count)++;
  sprintf(graphs,"%s/tmp/graphs%d/graph%d",directory,extension,(*graph_count));
  /*open file to save received graph data*/
  FILE* graphfile = fopen(graphs,"wb");
  if (graphfile == NULL)
   fprintf(stderr,"could not create graph file\n");

  search_graph_end(begingraph+strlen("<GRAPH>")+1,graphfile,dest,graph_count,extension,graphs,directory,
    have-(begingraph+strlen("<GRAPH>")+1-buf));
 }
 else
 {
  if (have > 1)
  xwrite(dest,buf,have);
  buf = NULL;
 }
    }

     void search_graph_end(char* buf, FILE* graphfile, FILE *dest,int* graph_count,int extension,
  char* graphs,char* directory,int have)
     {
 char* endgraph = NULL;
 endgraph = strstr(buf,"<GRAPH/>");
 if (endgraph != NULL)
 {
  xwrite(graphfile,buf,sizeof(char)*(endgraph-buf));
  fclose(graphfile);
  search_graph_begin(endgraph+strlen("<GRAPH/>")+1,dest,graph_count,extension,graphs,directory,
    have-(endgraph+strlen("<GRAPH/>")+1-buf));
 }
 else
 {
  if (have > 1)
  xwrite(graphfile,buf,have);
  buf = NULL;
 }
    }

the program runs fine under valgrind but its not the case when not. The idea of the program is to read in loop a buffer and write text between valise and in different files