Fix for XRef stream parsing when last object not included

[gunnsth]

Some PDF files using an XRef stream for cross-references have been encountered that don't specify the last object (in Index) and the last object is referring to the actual XRef stream itself.

The fix is twofold:

1. If the Index is missing coverage of the last object, instead of appending objNum+1, we append the maximumIndex+1 where maximumIndex is the maximum object number encountered prior to that object.
2. When we encounter an object in the table with an offset equal to where the XRef stream was parsed, then automatically correct the object number.

Test included which covers this case.

[codecov]

Codecov Report

Merging #399 into v3 will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              v3     #399      +/-   ##
=========================================
+ Coverage   60.8%   60.89%   +0.08%     
=========================================
  Files        151      151              
  Lines      27493    27502       +9     
=========================================
+ Hits       16716    16746      +30     
+ Misses     10374    10371       -3     
+ Partials     403      385      -18

Impacted Files	Coverage Δ
pdf/core/parser.go	76.09% <100%> (+0.62%)	⬆️
pdf/creator/styled_paragraph.go	93.57% <0%> (+3.33%)	⬆️
pdf/creator/utils.go	82.14% <0%> (+10.71%)	⬆️

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[adrg]

Looks good to me.