Fix test_python on Windows. After commit https://github.com/kripken/emscripten/commit/559c4f999b3dffda138f6e09cd068e2b662d9a3a , FS.lookupPath could return a JS object which has node==null, so test that as an error condition in readlink.
Merge pull request #2606 from juj/readlink_test_null

Fix test_python on Windows. After commit https://github.com/kripken/emsc...