Repair a botched PyOS_snprintf conversion.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 365a836..3340131 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -432,7 +432,7 @@
     if (rc == NO_ERROR)
         os2_formatmsg(msgbuf, msglen, reason);
     else
-        PyOS_snprintf(msgbuf, sizeof(msgbuf),
+        PyOS_snprintf(msgbuf, msgbuflen,
         	      "unknown OS error #%d", errorcode);
 
     return msgbuf;