fix various issues

- wrong typo in comments
- replace psk null check with key_exchange_mode check
- set psk NULL when error return in export hs psk

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index c0703a2..74dd40b 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1693,9 +1693,9 @@
 {
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     psa_key_attributes_t key_attributes = psa_key_attributes_init();
-    psa_status_t status;
+    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
     psa_algorithm_t alg = PSA_ALG_ANY_HASH;
-    mbedtls_svc_key_id_t key;
+    mbedtls_svc_key_id_t key = MBEDTLS_SVC_KEY_ID_INIT;
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
     if( psk == NULL || ssl->handshake == NULL )