Squashed commit upgrading to mbedtls-2.16.0

Squash merging branch import/mbedtls-2.16.0

9ee2a92de51f ("libmbedtls: compile new files added with 2.16.0")
9b0818d48d29 ("mbedtls: fix memory leak in mpi_miller_rabin()")
2d6644ee0bbe ("libmedtls: mpi_miller_rabin: increase count limit")
d831db4c238a ("libmbedtls: add mbedtls_mpi_init_mempool()")
df0f4886b663 ("libmbedtls: make mbedtls_mpi_mont*() available")
7b0792062b65 ("libmbedtls: refine mbedtls license header")
2616e2d9709f ("mbedtls: configure mbedtls to reach for config")
d686ab1c51b7 ("mbedtls: remove default include/mbedtls/config.h")
50a57cfac892 ("Import mbedtls-2.16.0")

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/mbedtls/library/ssl_cache.c b/lib/libmbedtls/mbedtls/library/ssl_cache.c
index b8130a6..4d8c866 100644
--- a/lib/libmbedtls/mbedtls/library/ssl_cache.c
+++ b/lib/libmbedtls/mbedtls/library/ssl_cache.c
@@ -321,6 +321,7 @@
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_free( &cache->mutex );
 #endif
+    cache->chain = NULL;
 }
 
 #endif /* MBEDTLS_SSL_CACHE_C */