sample: correct mbstowcs() error check

mbstowcs() can return (size_t)-1 if it encounters an invalid string

BUG=23200382

Change-Id: Ibf67145be3989e16cd24c06850c7a5aa581a0ada

sample.cpp

@@ -28,7 +28,7 @@ static const wchar_t* utf8towcs(const char* str) {
 
   const size_t size = mbstowcs(NULL, str, 0);
 
-  if (size == 0)
+  if (size == 0 || size == static_cast<size_t>(-1))
     return NULL;
 
   wchar_t* const val = new (std::nothrow) wchar_t[size + 1];