Fix to upstream bug

https://github.com/python-pillow/Pillow/pull/1060

Patch based on upstream commit to Pillow 2.7.0 (PIL fork)

https://github.com/wiredfool/Pillow/commit/44286ba3c9bfa6ed565d11bd61460d8ec215e1ea

Note that this patch includes a test of the fix, which requires an 

image file which is copied in from files/png_decompress_dos.png,

since it cannot be patched in.

--- Imaging-1.1.7-orig/PIL/PngImagePlugin.py	2015-01-21 17:45:12.000000000 -0800

+++ Imaging-1.1.7/PIL/PngImagePlugin.py	2015-01-21 19:37:23.000000000 -0800

@@ -68,6 +68,12 @@ _MODES = {

     (16,6): ("RGBA", "RGBA;16B"),

 }

+def _safe_zlib_decompress(s):

+    dobj = zlib.decompressobj()

+    plaintext = dobj.decompress(s, ImageFile.SAFEBLOCK)

+    if dobj.unconsumed_tail:

+        raise ValueError("Decompressed Data Too Large")

+    return plaintext

 # --------------------------------------------------------------------

 # Support classes.  Suitable for PNG and related formats like MNG etc.

@@ -197,7 +203,7 @@ class PngStream(ChunkStream):

         if comp_method != 0:

             raise SyntaxError("Unknown compression method %s in iCCP chunk" % comp_method)

         try:

-            icc_profile = zlib.decompress(s[i+2:])

+            icc_profile = _safe_zlib_decompress(s[i+2:])

         except zlib.error:

             icc_profile = None # FIXME

         self.im_info["icc_profile"] = icc_profile

@@ -293,7 +299,7 @@ class PngStream(ChunkStream):

         if comp_method != 0:

             raise SyntaxError("Unknown compression method %s in zTXt chunk" % comp_method)

         import zlib

-        self.im_info[k] = self.im_text[k] = zlib.decompress(v[1:])

+        self.im_info[k] = self.im_text[k] = _safe_zlib_decompress(v[1:])

         return s

 # --------------------------------------------------------------------

--- Imaging-1.1.7-orig/selftest.py	2015-01-21 17:44:51.000000000 -0800

+++ Imaging-1.1.7/selftest.py	2015-07-02 17:06:23.636751412 -0700

@@ -9,6 +9,7 @@ from PIL import Image

 from PIL import ImageDraw

 from PIL import ImageFilter

 from PIL import ImageMath

+from PIL import PngImagePlugin

 try:

     Image.core.ping

@@ -146,6 +147,15 @@ def testimage():

     >>> im.mode, im.size

     ('F', (128, 128))

+    Test fix to PNG decompression DOS #1060

+

+    >>> try:

+    ...  im = Image.open("Images/png_decompression_dos.png")

+    ...  im.load()

+    ... except ValueError as msg:

+    ...  print msg

+    Decompressed Data Too Large

+

     PIL can do many other things, but I'll leave that for another

     day.  If you're curious, check the handbook, available from: