Fix pg_upgrade's pg_scandir_internal() to properly handle a NULL

pattern, which is used on PG 9.1 and HEAD (but not pre-9.1).  Fixes
crash on Windows.

Backpatched to 9.1.

Reported by Mark Dilger

contrib/pg_upgrade/file.c

@@ -294,7 +294,7 @@ pg_scandir_internal(const char *dirname,
 	while ((direntry = readdir(dirdesc)) != NULL)
 	{
 		/* Invoke the selector function to see if the direntry matches */
-		if ((*selector) (direntry))
+		if (selector && (*selector) (direntry))
 		{
 			count++;