ssl_tls: Fix invalid buffer sizes during compression / decompression

Adjust information passed to zlib to include already written data.
diff --git a/ChangeLog b/ChangeLog
index ae8d86f..e15a53b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -55,6 +55,8 @@
      in the internal buffers; these cases lead to deadlocks in case
      event-driven I/O was used.
      Found and reported by Hubert Mis in #772.
+   * Fix invalid buffer sizes passed to zlib during record compression and
+     decompression.
 
 Changes
    * Remove some redundant code in bignum.c. Contributed by Alexey Skalozub.