libpng - security update

Several vulnerabilities have been discovered in the libpng PNG library.
The Common Vulnerabilities and Exposures project identifies the
following problems:

• CVE-2015-7981
  Qixue Xiao discovered an out-of-bounds read vulnerability in the
  png_convert_to_rfc1123 function. A remote attacker can potentially
  take advantage of this flaw to cause disclosure of information from
  process memory.
• CVE-2015-8126
  Multiple buffer overflows were discovered in the png_set_PLTE and
  png_get_PLTE functions. A remote attacker can take advantage of this
  flaw to cause a denial of service (application crash) via a small
  bit-depth value in an IHDR (image header) chunk in a PNG image.

For the oldstable distribution (wheezy), these problems have been fixed
in version 1.2.49-1+deb7u1.

For the stable distribution (jessie), these problems have been fixed in
version 1.2.50-2+deb8u1.

For the unstable distribution (sid), these problems have been fixed in
version 1.2.54-1.

We recommend that you upgrade your libpng packages.