Make freq unsigned

This value should never be less than 0

tree.h

@@ -11,7 +11,7 @@ struct tree {
 	struct tree *left;
 	struct tree *left;
 	struct tree *right;
 	struct tree *right;
 	unsigned char ch;
 	unsigned char ch;
-	int freq;
+	unsigned int freq;
 };
 };
 
 
 struct tree *insert_simple_tree(int character, int frequency);
 struct tree *insert_simple_tree(int character, int frequency);