On WIN32, don't choke when setlocale(LC_MESSAGES, "") returns NULL.

Per report from Magnus.

src/backend/utils/adt/pg_locale.c

@@ -4,7 +4,7 @@
  *
  * Portions Copyright (c) 2002-2003, PostgreSQL Global Development Group
  *
- * $PostgreSQL: pgsql/src/backend/utils/adt/pg_locale.c,v 1.25 2004/01/19 19:04:40 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/adt/pg_locale.c,v 1.26 2004/05/27 19:19:05 tgl Exp $
  *
  *-----------------------------------------------------------------------
  */
@@ -131,11 +131,22 @@ locale_messages_assign(const char *value, bool doit, GucSource source)
 	if (doit)
 	{
 		if (!setlocale(LC_MESSAGES, value))
+		{
+#ifdef WIN32
+			/*
+			 * Win32 returns NULL when you set LC_MESSAGES to "".  So don't
+			 * complain unless we're trying to set it to something else.
+			 */
+			if (value[0])
+				return NULL;
+#else
 			return NULL;
+#endif
+		}
 	}
 	else
 		value = locale_xxx_assign(LC_MESSAGES, value, false, source);
-#endif
+#endif /* LC_MESSAGES */
 	return value;
 }