Fix NULL handling in mbedtls_ssl_config.free() function

Signed-off-by: Troy-Butler <squintik@outlook.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index ac53853..32fa8e3 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -6029,6 +6029,10 @@
  */
 void mbedtls_ssl_config_free(mbedtls_ssl_config *conf)
 {
+    if (conf == NULL) {
+        return;
+    }
+
 #if defined(MBEDTLS_DHM_C)
     mbedtls_mpi_free(&conf->dhm_P);
     mbedtls_mpi_free(&conf->dhm_G);