Move 'format' into printbuffer

tests/print_number.c

@@ -28,7 +28,7 @@ static void assert_print_number(const char *expected, double input)
 {
     unsigned char printed[1024];
     cJSON item[1];
-    printbuffer buffer = { 0, 0, 0, 0, 0 };
+    printbuffer buffer = { 0, 0, 0, 0, 0, 0 };
     buffer.buffer = printed;
     buffer.length = sizeof(printed);
     buffer.offset = 0;