Some more nasty bugs fixed in the lf t55xx manchester_decode method.

ADD: a little function to see if GraphBuffer is not used.

client/graph.c

@@ -9,6 +9,7 @@
 //-----------------------------------------------------------------------------
 
 #include <stdio.h>
+#include <stdbool.h>
 #include <string.h>
 #include "ui.h"
 #include "graph.h"
@@ -93,3 +94,15 @@ int GetClock(const char *str, int peak, int verbose)
 
   return clock;
 }
+
+
+/* A simple test to see if there is any data inside Graphbuffer. 
+*/
+bool HasGraphData(){
+
+	if ( GraphTraceLen <= 0) {
+		PrintAndLog("No data available, try reading something first");
+		return false;
+	}
+	return true;	
+}