keyboard: Prevent input focus loss if no completion found

This prevents the command line input from losing focus when <kbd>Tab</kbd>
is hit, but there was no command completion found.

scripts/core/keyboard.js

@@ -17,9 +17,9 @@ function Keyboard()
 
     // Autocomplete with tab
     if(event.keyCode === 9){
+      event.preventDefault();
       var ac = ronin.terminal.find_autocomplete();
       if(ac){
-        event.preventDefault();
         ronin.terminal.input.value += ac;
       }
     }