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 0c247f9..eccf026 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -26,6 +26,8 @@
      the mbedtls_cipher_update() documentation. Contributed by Andy Leiserson.
    * Fix overriding and ignoring return values when parsing and writing to
      a file in pk_sign program. Found by kevlut in #1142.
+   * Fix invalid buffer sizes passed to zlib during record compression and
+     decompression.
 
 Changes
    * Improve testing in configurations that omit certain hashes or