imobiledevice: Check error returned by idevice_new
If the device we query for isn't known, the device pointer will
remain untouched according to idevice.c, and we leave it dangling.
Instead, check whether it returns IDEVICE_E_SUCCESS.
If the device we query for isn't known, the device pointer will
remain untouched according to idevice.c, and we leave it dangling.
Instead, check whether it returns IDEVICE_E_SUCCESS.