fix another printf("%s", NULL) case

ChangeLog

+Thu Jan 29 17:15:18 GMT 2009 John Levon <john.levon@sun.com>
+
+	* src/libvirt.c: fix another printf("%s", NULL) case
+
 Thu Jan 29 09:03:17 PST 2009 John Levon <john.levon@sun.com>
 
 	* src/xend_internal.c: correctly enable floppies for HVM guests.

src/libvirt.c

@@ -6924,7 +6924,7 @@ error:
 int
 virNodeNumOfDevices(virConnectPtr conn, const char *cap, unsigned int flags)
 {
-    DEBUG("conn=%p, cap=%s, flags=%d", conn, cap, flags);
+    DEBUG("conn=%p, cap=%s, flags=%d", conn, NULLSTR(cap), flags);
 
     virResetLastError();