fmt_fp.c: revert 647324ad45c0d779a3c36f0bb7955b8b8fc9f491

C++ complier does not allow jump across local variable declaration, even
if we don't access those variables after the destination label.

src/fmt_fp.c

@@ -118,7 +118,9 @@ mrb_format_float(mrb_float f, char *buf, size_t buf_size, char fmt, int prec, ch
       *s++ = 'N' ^ uc;
       *s++ = 'A' ^ uc;
       *s++ = 'N' ^ uc;
-      goto ret;
+    ret:
+      *s = '\0';
+      return (int)(s - buf);
     }
   }
 
@@ -363,7 +365,7 @@ mrb_format_float(mrb_float f, char *buf, size_t buf_size, char fmt, int prec, ch
     *s++ = '0' + (e / 10);
     *s++ = '0' + (e % 10);
   }
- ret:
+
   *s = '\0';
   return (int)(s - buf);
 }