Don't free an uninitalized pointer in update_driver_name()

This invalid free results in heap corruption. Some symptoms I saw
because of this were libvirtd crashing and virt-manager hanging
while trying to enumerate devices.

src/node_device/node_device_driver.c

@@ -78,7 +78,7 @@ static int update_driver_name(virConnectPtr conn,
                               virNodeDeviceObjPtr dev)
 {
     char *driver_link = NULL;
-    char *devpath;
+    char *devpath = NULL;
     char *p;
     int ret = -1;
 
@@ -114,7 +114,7 @@ static int update_driver_name(virConnectPtr conn,
 
 cleanup:
     VIR_FREE(driver_link);
-    free(devpath);
+    VIR_FREE(devpath);
     return ret;
 }
 #else