Fix SettingsSerializer not resetting after load

After loading SettingsSerializer would keep the last group/array as current, so setting new values before the next beginGroup/beginWriteArray would be added to this group/array wrongly

This has zero inpact on qTox since we don't load before saving

src/persistence/settingsserializer.cpp

@@ -401,6 +401,8 @@ void SettingsSerializer::readSerialized()
             //qDebug() <<"!Array end";
         }
     }
+
+    group = array = -1;
 }
 
 void SettingsSerializer::readIni()
@@ -545,6 +547,8 @@ void SettingsSerializer::readIni()
         //qDebug() << "Removing spurious array group"<<g<<groupSizes[g];
         removeGroup(g);
     }
+
+    group = array = -1;
 }
 
 void SettingsSerializer::removeGroup(int group)