Fix IllegalStateException on buffer readline

Fix this crash that was due to the kotlin migration:

021-07-05 07:18:47.236 6151-6151/org.videolan.vlcbenchmark.debug E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.videolan.vlcbenchmark.debug, PID: 6151
    java.lang.IllegalStateException: it must not be null
        at org.videolan.vlcbenchmark.SystemPropertiesProxy.readSysFile(SystemPropertiesProxy.kt:75)
        at org.videolan.vlcbenchmark.SystemPropertiesProxy.getCpuModel(SystemPropertiesProxy.kt:112)
        at org.videolan.vlcbenchmark.MainPageFragment.fillDeviceLayout(MainPageFragment.kt:141)
        at org.videolan.vlcbenchmark.MainPageFragment.onCreateView(MainPageFragment.kt:172)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2612)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:874)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1228)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1293)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2066)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1856)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1811)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1717)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2650)
        at androidx.fragment.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManagerImpl.java:2600)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:246)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:542)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:201)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1435)
        at android.app.Activity.performStart(Activity.java:8222)
        at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3818)
        at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221)
        at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2307)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8512)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)