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
```

lib/system/strs_v2.nim

@@ -112,9 +112,10 @@ proc nimToCStringConv(s: NimStringV2): cstring {.compilerproc, nonReloadable, in
 
 proc appendString(dest: var NimStringV2; src: NimStringV2) {.compilerproc, inline.} =
   if src.len > 0:
-    # also copy the \0 terminator:
-    copyMem(unsafeAddr dest.p.data[dest.len], unsafeAddr src.p.data[0], src.len+1)
+    # don't copy the \0 terminator:
+    copyMem(unsafeAddr dest.p.data[dest.len], unsafeAddr src.p.data[0], src.len)
     inc dest.len, src.len
+    dest.p.data[dest.len] = '\0'
 
 proc appendChar(dest: var NimStringV2; c: char) {.compilerproc, inline.} =
   dest.p.data[dest.len] = c