title: DEoptim stack imbalance problems
body: <p>When running the following optimization task (R v.3.0.2)</p>
  
  <pre><code>library(DEoptim)
  
  x &lt;- seq(-6,6,length=100); y &lt;- tanh(x)
  goal &lt;- function(par) return(1-abs(cor(x*par,y,method='spearman')))
  
  ctrl &lt;- DEoptim::DEoptim.control(VTR=0, trace=FALSE)
  
  res &lt;- DEoptim::DEoptim(goal,lower=-1,upper=1, ctrl)                  
  </code></pre>
  
  <p>I get stack imbalance warnings</p>
  
  <pre><code>Warning: stack imbalance in '&lt;-', 14 then 13
  Warning: stack imbalance in 'withVisible', 7 then 6
  </code></pre>
  
  <p>and <code>unprotect()</code> errors. If <code>VTR</code> is set below 0 (i.e. to a value impossible to obtain) then the problem disappears, but I'd rather not do that due to performance issues. </p>
  
  <p>The result gets returned despite the errors however I fear it might be unstable/incorrect. Any ideas how to solve this?</p>