PDFBox StackOverflowError on malformed files when merging

I supply 2 PDF files (certificate-of-analysis.pdf and questionario-sulle.pdf) which cause an exception if i try to merge them using PDFMergerUtility.

I've implemented code examples which run to the exceptions:
https://bitbucket.org/radistao/pdfbox-merge-bug/
or just take the same sources from the attached radistao-pdfbox-merge-bug.zip

Those 2 files are included into the repository (test/resources/).

Error stack trace for certificate-of-analysis.pdf:

• using addSource():

  java.lang.StackOverflowError
  at java.util.HashMap.hash(HashMap.java:338)
  at java.util.HashMap.containsKey(HashMap.java:595)
  at java.util.HashSet.contains(HashSet.java:203)
  at org.apache.pdfbox.pdfwriter.COSWriter.addObjectToWrite(COSWriter.java:472)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromArray(COSWriter.java:927)
  at org.apache.pdfbox.cos.COSArray.accept(COSArray.java:515)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1061)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1020)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1020)

• using appendDocument()

  org.apache.pdfbox.cos.COSDocument finalize
  WARNING: Warning: You did not close a PDF Document
  org.apache.pdfbox.cos.COSDocument finalize
  WARNING: Warning: You did not close a PDF Document
  org.apache.pdfbox.cos.COSDocument finalize
  WARNING: Warning: You did not close a PDF Document
  org.apache.pdfbox.cos.COSDocument finalize
  WARNING: Warning: You did not close a PDF Document
  org.apache.pdfbox.cos.COSDocument finalize
  WARNING: Warning: You did not close a PDF Document

  java.lang.StackOverflowError
  at org.apache.pdfbox.pdfwriter.COSWriter.getObjectKey(COSWriter.java:892)
  at org.apache.pdfbox.pdfwriter.COSWriter.writeReference(COSWriter.java:1175)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromArray(COSWriter.java:928)
  at org.apache.pdfbox.cos.COSArray.accept(COSArray.java:515)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1061)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1020)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1020)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1020)
  at org.apache.pdfbox.pdfwriter.COSWriter.visitFromDictionary(COSWriter.java:1020)

Note: in version 1.8.13 (see https://bitbucket.org/radistao/pdfbox-merge-bug/branch/test/v1.8.13) file questionario-sulle.pdf merged fine (see attached merge result v1.8.13-PdfMergerAddSourceTest-questionario-sulle.pdf), but with just warnings:

org.apache.pdfbox.pdfparser.PDFParser parseObject
WARNING: expected='%%EOF' actual=''
org.apache.pdfbox.pdfparser.XrefTrailerResolver setStartxref
WARNING: Did not found XRef object at specified startxref position 0