idevicebackup2: Fix logical bug when checking for success of backup operation
idevicebackup2: Fix logical bug when checking for success of backup operation

The condition in line 2278 is incorrectly evaluated when
mb2_status_check_snapshot_state() isn't able to read the Status.plist file.
While `if (-1) { ... }` will be a 'false' condition, `if (1 && -1) { ... }`
will be 'true' which in this case would make idevicebackup2 assume the backup
was successful while it was not.
This commit fixes this issue by changing the default return value of
mb2_status_check_snapshot_state() to be 0 (false).

Thanks to Xiao Deng for pointing out this issue!

libimobiledevice-win32/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,Tatsh/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice-win32/libimobiledevice