Merge to XFA: Remove dead code from CPDF_Metadata. Add missing nullptr check. BUG=pdfium:117 R=tsepez@chromium.org Review URL: https://codereview.chromium.org/1282653002 . (cherry picked from commit 1e2ece0cf98f8daecdea5b45ebd20cedbcef4bd9) Review URL: https://codereview.chromium.org/1274803004 .