[Vserve+V4RB] several cursor.updates in a go

[Eric Ferrer]

> Before create new cursor kill the prev
> 
>     cursor = nil

Thank you Ruslan,

This trick allows me to click on this test button up to 6/7 times... Then
the app crashes again.
This is better than crashing at this first click, but this problem does not
reassure me.

I changed the code in my test button, and it now updates a varchar(256)
field 3 times in a go, and I have absolutely no problem. I can click on it
dozens of times, even without the 'cursor = nil' trick : the app and
Valentina update the records normally.

I think there is something wrong with text-typed fields.

Maybe I should reconsider my database's structure, but for sure, I have
cases where text-types fields cannot be replaced, as far as I know.

Thanks again
Eric