Icns DOS fix -- CVE-2014-3589

Found and reported by Andrew Drake of dropbox.com

PIL/IcnsImagePlugin.py

@@ -120,6 +120,8 @@
         i = HEADERSIZE
         while i < filesize:
             sig, blocksize = nextheader(fobj)
+            if blocksize <= 0:
+                raise SyntaxError('invalid block header')
             i = i + HEADERSIZE
             blocksize = blocksize - HEADERSIZE
             dct[sig] = (i, blocksize)

Tests/check_icns_dos.py

+# Tests potential DOS of IcnsImagePlugin with 0 length block.
+# Run from anywhere that PIL is importable. 
+
+from PIL import Image
+from io import BytesIO
+
+if bytes is str:
+    Image.open(BytesIO(bytes('icns\x00\x00\x00\x10hang\x00\x00\x00\x00')))
+else:
+    Image.open(BytesIO(bytes('icns\x00\x00\x00\x10hang\x00\x00\x00\x00', 'latin-1')))