Prevent creating window functions with default arguments.

Insertion of default arguments doesn't work for window functions, which is
likely to cause a crash at runtime if the implementation code doesn't check
the number of actual arguments carefully.  It doesn't seem worth working
harder than this for pre-9.2 branches.

src/backend/catalog/pg_proc.c

@@ -284,6 +284,12 @@ ProcedureCreate(const char *procedureName,
 		}
 	}
 
+	/* Guard against a case the planner doesn't handle yet */
+	if (isWindowFunc && parameterDefaults != NIL)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("window functions cannot have default arguments")));
+
 	/*
 	 * All seems OK; prepare the data to be inserted into pg_proc.
 	 */