fixes #24664; always sets the \0 terminator in appendString (#24703)

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
```

tests/system/t24664.nim

@@ -0,0 +1,14 @@
+discard """
+  output: '''
+TestString123TestString123
+TestString123TestString123
+'''
+"""
+
+proc foostring() = # bug #24664
+  for i in 0..1:
+    var s = "TestString123"
+    s.add s
+    echo s
+
+foostring()