Fix JBig2 decoding logic
Fix JBig2 decoding logic

While decoding among instances, variable "FIRSTS" should hold its
value, not be reset. This was accidently changed by earlier
refactoring.

BUG=chromium:625848,pdfium:636

Review-Url: https://codereview.chromium.org/2569023002

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium