fix MSVC initialization issue by a patch from @dyama; close #2734

src/fmt_fp.c

@@ -360,7 +360,10 @@ fmt_core(struct fmt_args *f, const char *fmt, mrb_float flo)
 mrb_value
 mrb_float_to_str(mrb_state *mrb, mrb_value flo, const char *fmt)
 {
-  struct fmt_args f = { mrb, mrb_str_buf_new(mrb, 24) };
+  struct fmt_args f;
+
+  f.mrb = mrb;
+  f.str = mrb_str_buf_new(mrb, 24);
   if (fmt_core(&f, fmt, mrb_float(flo)) < 0) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "invalid format string");
   }