Show error when we do not support the file

Our special 0xff as EOF does not allow some files.

tree.c

@@ -37,6 +37,11 @@ struct tree *create_tree(FILE *file)
 		if (c == EOF || c > MY_BUF-1)
 			break;
 
+		if (c > MY_BUF-1 || c == EOF_CHAR) {
+			fprintf(stderr, "File not supported\n");
+			return NULL;
+		}
+
 		if (freq[c] == 0)
 			charcount++;