This is a fix for issues related to:
'undefined method `[]' for nil:NilClass'
It is possible that a PDF may not have the 'Size' xref, and people
are running into the 'undefined method'[]' for NilClass' exception.
Because the pdf parser always assumes there is a Size field,
so it uses a match() function to find the value for Size, which
can be nil.
See the following bug report for example:
https://dev.metasploit.com/redmine/issues/7014
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