postgres_fdw: Add Assert to estimate_path_cost_size().

postgres_fdw: Add Assert to estimate_path_cost_size().

When estimating the cost/size of a pre-sorted path for a given upper
relation using local stats, this function dereferences the passed-in
PgFdwPathExtraData pointer without checking that it is not NULL.  But
that is not a bug as the pointer is guaranteed to be non-NULL in that
case; to avoid confusion, add an Assert to ensure that it is not NULL
before dereferencing it.

Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/CAEudQArgiALbV1akQpeZOgim7XP05n%3DbDP1%3DTcOYLA43nRX_vA%40mail.gmail.com