Fix unused-but-set-variable compiler warning in reorderbuffer.c.

 Fix unused-but-set-variable compiler warning in reorderbuffer.c.
 
 On v13, this variable is only used for an assertion, so adding
 PG_USED_FOR_ASSERTS_ONLY is sufficient to suppress this warning on
 builds with assertions disabled.  Older versions are unsupported,
 and newer versions use the variable for more than the assertion, so
 this patch only needs to be applied to REL_13_STABLE.
 
 Reviewed-by: Tom Lane
 Discussion: https://postgr.es/m/Z1dCFnzrP24O8WNR%40nathan