fixes#24664
```nim
proc main() =
for i in 0..1:
var s = "12345"
s.add s
echo s
main()
```
In the given example, `add` contains two steps: `prepareAdd` and
`appendString`. In the first step, a new buffer is created in order to
store the final doubled string. But it doesn't copy the null terminator,
neither zeromem the left unused spaces. It causes a problem because
`appendString` will copy itself which doesn't end with `\0` properly so
contaminated memory is copied instead.
```
var s = 12345\0
prepareAdd:
var s = 12345xxxxx\0
appendString:
var s = 1234512345x
```
1f07fdd2dc