Fix potential double-free in ssl_set_psk()

Internal ref: IOTSSL-517
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7fc9d99..166b116 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4058,6 +4058,8 @@
     {
         polarssl_free( ssl->psk );
         polarssl_free( ssl->psk_identity );
+        ssl->psk = NULL;
+        ssl->psk_identity = NULL;
     }
 
     if( ( ssl->psk = polarssl_malloc( psk_len ) ) == NULL ||