[AC21.4] Arnaud Giersch's patch for regex.c / fix Gnus infloop

RECOMMEND 21.4 Not needed for 21.5 (it's already in 21.5; this has been a "forehead, meet palm" experience :-P ). This is Arnaud Giersch's patch to fix the Gnus infloop caused by my errors in backporting the 21.5 shy groups code. Andrey, you were looking at almost the right place! I'd appreciate it if you could check, too, although Arnaud is surely looking at the same infloop that you reported. Besides being in 21.5 and theoretically correct, Arnaud says it fixes the infloop for him. And it's a fencepost-like error, as I suggested. I'm sure this is it, although I can't test easily. This _might_ fix _some_ of the "wrong side of point" errors and so on: they could be due to accessing stale match register data, which is what this patch prevents. But I think some of those were previous to application of the backport of 21.5's shy groups, so don't get your hopes up too high. Thanks, Arnaud! I've added the ChangeLog, interchanged the initialization and the comment at the top of the block, and regularized the whitespace with `indent-region' (it's all tabs, now). Sorry, Arnaud, this will cause a conflict when you CVS update.