Fix another invalidated iterator issue while traversing CPDF_Dictionary.
Fix another invalidated iterator issue while traversing CPDF_Dictionary.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1582903004 .

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