Add missing va_end before return

PR-URL: https://github.com/nodejs/node/pull/3565
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node.cc

@@ -176,6 +176,7 @@ static void PrintErrorString(const char* format, ...) {
       stderr_handle == nullptr ||
       uv_guess_handle(_fileno(stderr)) != UV_TTY) {
     vfprintf(stderr, format, ap);
+    va_end(ap);
     return;
   }