getTtsLanguage: workaround NPE in >= Lollipop.

When there is no TTS engine selected, mTts.getVoice() will throw
a NullPointerException. This is a workaround to cope with this bug
in Android.

android/src/com/reecedunn/espeak/eSpeakActivity.java

     private Locale getTtsLanguage() {
         if (mTts != null) {
             if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
-                android.speech.tts.Voice voice = mTts.getVoice();
+                android.speech.tts.Voice voice = null;
+                try {
+                    voice = mTts.getVoice();
+                } catch (NullPointerException e) {
+                    // Android Lollipop and later may throw a NullPointerException
+                    // if a voice is not selected.
+                    return null;
+                }
                 if (voice != null) {
                     return voice.getLocale();
                 }