Proper define of NDEBUG to avoid adding assertions in string benchmarks

bench/bench-string-2.cpp

@@ -22,6 +22,8 @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
+#define NDEBUG
+
 #include <string>
 #include <sstream>
 #include <string>

bench/bench-string.cpp

@@ -48,11 +48,12 @@
  *
  */
 
+#define NDEBUG
+
 #include <stdio.h>
 #include <time.h>
 #include <limits.h>
 
-#define NDEBUG
 #define M_USE_SMALL_NAME 0
 #include "common.h"