2.5-stable: pdf: fix failure exporting issue pdf with UTF-8 U+FFFD (#17826)

This revision has trunk r13368 and r13369 test.

git-svn-id: http://svn.redmine.org/redmine/branches/2.5-stable@13370 e93f8b46-1217-0410-a6f0-8f06a7374b81