fix a NULL pointer dereference
fix a NULL pointer dereference

This commit fixes a bug that never bit.

The probe_new() function makes a window that lets the user select the
HAL object to display.  It takes a string argument intended to be the
title of the selection window.  The function intends to check the string
for NULL, and supply a default window title if the caller didn't provide
one, but it got the logic of the test reversed: if the caller provided
a title it always got replaced with the default, and if no title was
provided it would allow the NULL string through.

This bug never bit because probe_new() is only ever called by meter_new(),
which supplies a window title.

bobvanderlinden/machinekit,cdsteinkuehler/linuxcnc,kinsamanka/machinekit,cdsteinkuehler/MachineKit,cnc-club/linuxcnc,ArcEye/MK-Qt5,EqAfrica/machinekit,ArcEye/machinekit-testing,ianmcmahon/linuxcnc-mirror,bobvanderlinden/machinekit,strahlex/machinekit,ArcEye/MK-Qt5,strahlex/machinekit,araisrobo/machinekit,cdsteinkuehler/MachineKit,RunningLight/machinekit,bmwiedemann/linuxcnc-mirror,araisrobo/machinekit,Cid427/machinekit,strahlex/machinekit,bmwiedemann/linuxcnc-mirror,RunningLight/machinekit,ianmcmahon/linuxcnc-mirror,ArcEye/machinekit-testing,strahlex/machinekit,unseenlaser/machinekit,mhaberler/machinekit,mhaberler/machinekit,araisrobo/machinekit,aschiffler/linuxcnc,ikcalB/linuxcnc-mirror,EqAfrica/machinekit,unseenlaser/machinekit,araisrobo/linuxcnc,Cid427/machinekit,RunningLight/machinekit,cdsteinkuehler/MachineKit,RunningLight/machinekit,araisrobo/machinekit,bmwiedemann/linuxcnc-mirror,unseenlaser/linuxcnc,strahlex/machinekit,EqAfrica/machinekit,cdsteinkuehler/MachineKit,cdsteinkuehler/MachineKit,RunningLight/machinekit,ianmcmahon/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bobvanderlinden/machinekit,cnc-club/linuxcnc,mhaberler/machinekit,Cid427/machinekit,RunningLight/machinekit,mhaberler/machinekit,ArcEye/MK-Qt5,cnc-club/linuxcnc,kinsamanka/machinekit,EqAfrica/machinekit,ikcalB/linuxcnc-mirror,ArcEye/machinekit-testing,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,araisrobo/linuxcnc,narogon/linuxcnc,araisrobo/machinekit,EqAfrica/machinekit,mhaberler/machinekit,unseenlaser/machinekit,cnc-club/linuxcnc,kinsamanka/machinekit,araisrobo/machinekit,RunningLight/machinekit,ianmcmahon/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,mhaberler/machinekit,bmwiedemann/linuxcnc-mirror,ArcEye/machinekit-testing,unseenlaser/machinekit,cdsteinkuehler/linuxcnc,EqAfrica/machinekit,Cid427/machinekit,ianmcmahon/linuxcnc-mirror,araisrobo/linuxcnc,Cid427/machinekit,strahlex/machinekit,Cid427/machinekit,bobvanderlinden/machinekit,araisrobo/linuxcnc,cdsteinkuehler/linuxcnc,cnc-club/linuxcnc,unseenlaser/machinekit,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,cnc-club/linuxcnc,jaguarcat79/ILC-with-LinuxCNC,mhaberler/machinekit,jaguarcat79/ILC-with-LinuxCNC,aschiffler/linuxcnc,araisrobo/machinekit,ArcEye/machinekit-testing,ArcEye/MK-Qt5,cnc-club/linuxcnc,aschiffler/linuxcnc,cdsteinkuehler/linuxcnc,narogon/linuxcnc,Cid427/machinekit,jaguarcat79/ILC-with-LinuxCNC,ArcEye/machinekit-testing,unseenlaser/machinekit,kinsamanka/machinekit,narogon/linuxcnc,araisrobo/linuxcnc,mhaberler/machinekit,ArcEye/MK-Qt5,bobvanderlinden/machinekit,jaguarcat79/ILC-with-LinuxCNC,kinsamanka/machinekit,kinsamanka/machinekit,unseenlaser/linuxcnc,ArcEye/machinekit-testing,Cid427/machinekit,ianmcmahon/linuxcnc-mirror,kinsamanka/machinekit,unseenlaser/linuxcnc,EqAfrica/machinekit,strahlex/machinekit,unseenlaser/linuxcnc,ikcalB/linuxcnc-mirror,unseenlaser/machinekit,ArcEye/MK-Qt5,aschiffler/linuxcnc,araisrobo/machinekit,kinsamanka/machinekit,cdsteinkuehler/linuxcnc,narogon/linuxcnc,bobvanderlinden/machinekit,ArcEye/machinekit-testing,narogon/linuxcnc,EqAfrica/machinekit,araisrobo/machinekit,bobvanderlinden/machinekit,aschiffler/linuxcnc,ArcEye/MK-Qt5,unseenlaser/machinekit,unseenlaser/linuxcnc,jaguarcat79/ILC-with-LinuxCNC,cdsteinkuehler/MachineKit,ianmcmahon/linuxcnc-mirror,cdsteinkuehler/linuxcnc,ArcEye/MK-Qt5,RunningLight/machinekit,bobvanderlinden/machinekit