Avoid problems when required twice

[vbfox]

It's possible due to test runners like jest playing with the node runtime object.

The native module can't be required twice but the javascript one can. As the javascript one return a mutated version of the native one it create problems :

exports._writePointer = exports.writePointer

exports.writePointer = function writePointer (buf, offset, ptr) {
  debug('writing pointer to buffer', buf, offset, ptr)
  exports._writePointer(buf, offset, ptr)
  exports._attach(buf, ptr)
}

After 2+ executions of this code writePointer would call itself and produce a stack overflow exception.

Additional info

• The problematic function in jest is : resetModules.
• I encountered the problem in the context of this PR to yarn : https://github.com/yarnpkg/yarn/pull/2960