pgsql: Make fixed_paramref_hook behave properly when there are 'unused'

Log Message:
-----------
Make fixed_paramref_hook behave properly when there are 'unused' slots
in the parameter array. Noted while experimenting with an example
from Pavel. This wouldn't come up in normal use, but it ought to honor
the specification that a parameter array can have unused slots.

Modified Files:
--------------
pgsql/src/backend/parser:
parse_param.c (r2.2 -> r2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_param.c?r1=2.2&r2=2.3)