Fix assertion failure in generate_orderedappend_paths()

 Fix assertion failure in generate_orderedappend_paths()
 
 In generate_orderedappend_paths(), there is an assumption that a child
 relation's row estimate is always greater than zero.  There is an
 Assert verifying this assumption, and the estimate is also used to
 convert an absolute tuple count into a fraction.
 
 However, this assumption is not always valid -- for example, upper
 relations can have their row estimates unset, resulting in a value of
 zero.  This can cause an assertion failure in debug builds or lead to
 the tuple fraction being computed as infinity in production builds.
 
 To fix, use the row estimate from the cheapest_total path to compute
 the tuple fraction.  The row estimate in this path should already have
 been forced to a valid value.
 
 In passing, update the comment for generate_orderedappend_paths() to
 note that the function also considers the cheapest-fractional case
 when not all tuples need to be retrieved.  That is, it collects all
 the cheapest fractional paths and builds an ordered append path for
 each interesting ordering.
 
 Backpatch to v18, where this issue was introduced.
 
 Bug: #19102
 Reported-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
 Author: Richard Guo <guofenglinux@gmail.com>
 Reviewed-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
 Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
 Discussion: https://postgr.es/m/19102-93480667e1200169@postgresql.org
 Backpatch-through: 18