FIX basket feature extraction with missing PTB data

Feature extraction of basket features crashes in the absence of corresponding PTB tokens and trees.

@kowey I has some trouble finding this bug, so you might want to reproduce it on your system to confirm the bug beforehand.

I was running  ̀irit-rst-dt gather` with the default parameters, e.g.
`TRAINING_CORPORA = ['corpus/RSTtrees-WSJ-double-1.0']`.
Failure was happening on `file4`, which goes by another name in the PTB.