core: Make ot_transfer_out_value() take a & for the second argument

As Ray Strode argued, it's confusing to have something that looks like
a function be magical.  And OT_TRANSFER_OUT_VALUE is uglier.

src/libotutil/ot-gio-utils.c

@@ -188,7 +188,7 @@ ot_gio_splice_and_checksum (GOutputStream  *out,
     }
 
   ret = TRUE;
-  ot_transfer_out_value(out_checksum, ret_checksum);
+  ot_transfer_out_value(out_checksum, &ret_checksum);
  out:
   ot_clear_checksum (&ret_checksum);
   return ret;