exec/execFile failure leaks file descriptors

[piscisaureus]

Reproducible on both v0.10 and v0.11

Test code:

var execFile = require('child_process').execFile;

var left = 1000;
exec();

function exec() {
  execFile('in%val#id', [], afterExec);
}

function afterExec() {
  if (--left)
    exec();
  else
    report();
}

function report() {
  console.log(process._getActiveHandles());
}