FlateStream: Verify Adler-32 checksum

A variant of the PDFex CBC gadget attack modifies Deflate compressed
data to assist with the creation of CBC gadgets. The attacker can use
CBC gadgets to prefix a URL to an encrypted plaintext, to create a
hyperlink which will leak decrypted data when clicked. However,
modifying the deflate data results in several random bytes in the
decompressed data, so we can limit this attack by verifying the Deflate
data's Adler-32 checksum.

See https://pdf-insecurity.org/encryption/encryption.html

Issue #814
6 jobs for adler in 9 minutes and 33 seconds (queued for 5 minutes and 25 seconds)
latest
Status Job ID Name Coverage
  Build
passed #1001629
build

00:03:23

passed #1001634
build_android

00:01:31

passed #1001630
build_clang_libcpp

00:04:17

passed #1001633
build_clazy

00:03:46

passed #1001632
build_mingw64_fedora

00:02:51

passed #1001631
build_ubuntu_16_04

00:03:27