child process: don't send signal if process is already terminated

Fixes failing test test/simple/test-exec-max-buffer.js

lib/child_process_uv.js

@@ -332,7 +332,7 @@ ChildProcess.prototype.kill = function(sig) {
     throw new Error('Unknown signal: ' + sig);
   }
 
-  if (!this.signalCode && !this.exitCode) {
+  if (this._internal) {
     var r = this._internal.kill(signal);
     // TODO: raise error if r == -1?
   }