Fix hmac_drbg failure in benchmark, with threading

Remove redunadnat calls to `hmac_drbg_free()` between seeding operations,
which make the mutex invalid. Fixes #1095
diff --git a/ChangeLog b/ChangeLog
index 44533d2..5e00c0e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,6 +22,8 @@
 Changes
    * Change the shebang line in Perl scripts to look up perl in the PATH.
      Contributed by fbrosson.
+   * Fix efailure in hmac_drbg in the benchmark sample application, when
+     MBEDTLS_THREADING_C is defined. Found by TrinityTonic, #1095
 
 = mbed TLS 2.11.0 branch released 2018-06-18