Fixed a potential null pointer dereference in png_set_text_2() (bug report
and patch by Patrick Keshishian).
https://nvd.nist.gov/vuln/detail/CVE-2016-10087
Change-Id: Ie90ccd0772335cbf0715bfd6da0a3d5c1b6eff99 Signed-off-by: Jiyong Min <jiyong.min@samsung.com>