Hotfix for restore position

[takahirom]

Overview (Required)

• I fixed two crashes.
• There is two patterns

                                                 Process: io.github.droidkaigi.confsched2018.debug, PID: 19218
                                                                                          java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
                                                                                              at java.util.ArrayList.get(ArrayList.java:411)
                                                                                              at io.github.droidkaigi.confsched2018.presentation.sessions.SessionsViewPagerAdapter.getItem(SessionsFragment.kt:282)
                                                                                              at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:109)
                                                                                              at io.github.droidkaigi.confsched2018.presentation.sessions.SessionsFragment.saveCurrentSession(SessionsFragment.kt:174)
                                                                                              at io.github.droidkaigi.confsched2018.presentation.sessions.SessionsFragment.onPause(SessionsFragment.kt:162)
                                                                                              at android.support.v4.app.Fragment.performPause(Fragment.java:2449)

[konifar]

LGTM!