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.

src/backend/parser/parse_param.c

@@ -17,7 +17,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/parse_param.c,v 2.2 2010/01/02 16:57:50 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/parse_param.c,v 2.3 2010/01/13 01:17:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -100,8 +100,9 @@ fixed_paramref_hook(ParseState *pstate, ParamRef *pref)
 	int			paramno = pref->number;
 	Param	   *param;
 
-	/* Check parameter number is in range */
-	if (paramno <= 0 || paramno > parstate->numParams)
+	/* Check parameter number is valid */
+	if (paramno <= 0 || paramno > parstate->numParams ||
+		!OidIsValid(parstate->paramTypes[paramno - 1]))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_PARAMETER),
 				 errmsg("there is no parameter $%d", paramno),