BUG: Fixes to FixedForwardWindowIndexer and GroupbyIndexer (#43267)
- [x] closes https://github.com/pandas-dev/pandas/issues/43267

This PR addresses three issues, two major and one minor:

(1) If you repeated the use of a FixedForwardWindowIndexer, the indexer would be mutated and a window_size of 0 would be used.  This would lead to an empty end array which unsurprisingly led to segfaults on the second run.  Similar issues could be produced with other BaseIndexer subclasses, this one was simply the loudest.

(2) The end array generated by FixedForwardWindowIndexer.get_window_bounds was the wrong length, leading to longer arrays than necessary.  When there was only one involved, it didn't matter much: the extra ones were simply ignored.  But in the case of a groupby, these end arrays were concatenated, meaning that end values for the first group could leak into the second, and so on.

(3) There were some typos for "indices". 