Improve I/O responsivness

Added checking of (stdin->_cnt > 0) from Greko.

This seems to greatly improve responsivness when running
under console. Now while running a 'stockfish bench', any key
press immediately is detected by SF while before there was a
delay of some fraction of a second.

No functional change.

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

src/search.cpp

@@ -2013,7 +2013,7 @@ split_point_start: // At split points actual search starts from here
     int t = current_search_time();
 
     //  Poll for input
-    if (data_available())
+    if (input_available())
     {
         // We are line oriented, don't read single chars
         std::string command;