[JBIG2] Fix getting decodeParms when it's an array

According to the specification, `DecodeParms` can be either a dictionary or an array of dictionaries: http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G6.1919337.

This patch handles the case where `DecodeParms` is an array with only one element. Obviously this will not work perfectly when there are multiple dictionaries present, but without a testcase it seemed easier to just address this special case and trigger the fallback bar otherwise.  

Fixes #3999.