Cannot configure "longer" mqtt topic than previously configured

[cakoolen]

Describe the bug
When I want to configure a longer mqtt topic name than previously configured, I cannot add the additional characters. So if my topic was co2 I cannot change it to co2-sensor for example.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Configuration/MQTT Config'
2. Scroll down to 'Topic'
3. Change 'Topic' to 'co2'
4. Save configuration
5. Go back to 'Configuration/MQTT Config'
6. Scroll down to 'Topic'
7. Change 'Topic' to 'co2-sensor'
8. It is not possible to add the - character.

Expected behavior
It should be possible to add characters after the last character of the previous setting.

Additional context
Looking at the code, the problem might be related with truncating the tempstring before copying it to the used value here: https://github.com/melkati/CO2-Gadget/blob/master/CO2_Gadget_Menu.h#L383

Note that this pattern is used for most settings.

[melkati]

I was able to replicate the issue.

As a workaround, until there is a fix, simply exit the menu and enter it again, then you should be able to input a longer topic. Make sure to be using latest release.