[cpp] weird crash/corruption with Dynamic and Visual Studio 2015

The following crashes for me on Windows if compiled with VS2015 (aka version 19 aka version 14). @Simn couldn't reproduce it with the previous one (`Using MSVC version: 18`). Interestingly, if we replace those `"hello"` strings with empty strings (`""`) it doesn't crash, but prints some rubbish instead.

``` haxe
class Main {
static function f(v:Dynamic) {
f2("hello" + Std.string(v) + "hello");
}

@:pure(false) static function f2(v:String) {
trace(v);
}

static function main(){
f("s");
}
}
```