Fix bench with fen files regression

Erroneusly adds default positions to the fens
loaded from external file.

Bug introduced in adb71b8096

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/benchmark.cpp

@@ -85,6 +85,7 @@ void benchmark(istringstream& is) {
 
   if (fenFile != "default")
   {
+      fens.clear();
       string fen;
       ifstream file(fenFile.c_str());