Repair incorrect check for coercion of unknown literal to ANYARRAY, a bug

I introduced in 7.4.1 :-(.  It's correct to allow unknown to be coerced to
ANY or ANYELEMENT, since it's a real-enough data type, but it most certainly
isn't an array datatype.  This can cause a backend crash but AFAICT is not
exploitable as a security hole.  Per report from Michael Fuhr.

Note: as fixed in HEAD, this changes a constant in the pg_stats view,
resulting in a change in the expected regression outputs.  The back-branch
patches have been hacked to avoid that, so that pre-existing installations
won't start failing their regression tests.

src/backend/parser/parse_coerce.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/parse_coerce.c,v 2.144 2006/10/04 00:29:55 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/parse_coerce.c,v 2.145 2006/10/11 20:21:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -129,11 +129,22 @@ coerce_type(ParseState *pstate, Node *node,
 		return node;
 	}
 	if (targetTypeId == ANYOID ||
-		targetTypeId == ANYARRAYOID ||
-		targetTypeId == ANYELEMENTOID)
+		targetTypeId == ANYELEMENTOID ||
+		(targetTypeId == ANYARRAYOID && inputTypeId != UNKNOWNOID))
 	{
-		/* assume can_coerce_type verified that implicit coercion is okay */
-		/* NB: we do NOT want a RelabelType here */
+		/*
+		 * Assume can_coerce_type verified that implicit coercion is okay.
+		 *
+		 * Note: by returning the unmodified node here, we are saying that
+		 * it's OK to treat an UNKNOWN constant as a valid input for a
+		 * function accepting ANY or ANYELEMENT.  This should be all right,
+		 * since an UNKNOWN value is still a perfectly valid Datum.  However
+		 * an UNKNOWN value is definitely *not* an array, and so we mustn't
+		 * accept it for ANYARRAY.  (Instead, we will call anyarray_in below,
+		 * which will produce an error.)
+		 *
+		 * NB: we do NOT want a RelabelType here.
+		 */
 		return node;
 	}
 	if (inputTypeId == UNKNOWNOID && IsA(node, Const))