gnu: libtasn1: Replace 4.7 with 4.8 [fixes CVE-2016-4008].

* gnu/packages/tls.scm (libtasn1)[replacement]: New field.
(libtasn1/fixed): New variable.

gnu/packages/tls.scm

@@ -65,8 +65,22 @@
 for transmitting machine-neutral encodings of data objects in computer
 networking, allowing for formal validation of data according to some
 specifications.")
+    (replacement libtasn1/fixed)
     (license license:lgpl2.0+)))
 
+(define libtasn1/fixed                            ;for CVE-2016-4008
+  (package
+    (inherit libtasn1)
+    (source
+     (let ((version "4.8"))
+       (origin
+         (method url-fetch)
+         (uri (string-append "mirror://gnu/libtasn1/libtasn1-"
+                             version ".tar.gz"))
+         (sha256
+          (base32
+           "04y5m29pqmvkfdbppmsdifyx89v8xclxzklpfc7a1fkr9p4jz07s")))))))
+
 (define-public p11-kit
   (package
     (name "p11-kit")