remove psk key when ephemeral selected

ephemeral is selected, `handshake->psk` must be removed.
Otherwise the encrypt key will be caculate fail.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index fe38a09..1cda3a7 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1662,7 +1662,7 @@
     return( ret );
 }
 
-static void ssl_remove_psk( mbedtls_ssl_context *ssl )
+void mbedtls_ssl_remove_psk( mbedtls_ssl_context *ssl )
 {
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     if( ! mbedtls_svc_key_id_is_null( ssl->handshake->psk_opaque ) )
@@ -1682,6 +1682,7 @@
         mbedtls_platform_zeroize( ssl->handshake->psk,
                                   ssl->handshake->psk_len );
         mbedtls_free( ssl->handshake->psk );
+        ssl->handshake->psk = NULL;
         ssl->handshake->psk_len = 0;
     }
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
@@ -1703,7 +1704,7 @@
     if( psk_len > MBEDTLS_PSK_MAX_LEN )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
 
-    ssl_remove_psk( ssl );
+    mbedtls_ssl_remove_psk( ssl );
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
 #if defined(MBEDTLS_SSL_PROTO_TLS1_2)
@@ -1780,7 +1781,7 @@
         ( ssl->handshake == NULL ) )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
 
-    ssl_remove_psk( ssl );
+    mbedtls_ssl_remove_psk( ssl );
     ssl->handshake->psk_opaque = psk;
     return( 0 );
 }
@@ -3522,25 +3523,7 @@
 #endif
 
 #if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
-#if defined(MBEDTLS_USE_PSA_CRYPTO)
-    if( ! mbedtls_svc_key_id_is_null( ssl->handshake->psk_opaque ) )
-    {
-        /* The maintenance of the external PSK key slot is the
-         * user's responsibility. */
-        if( ssl->handshake->psk_opaque_is_internal )
-        {
-            psa_destroy_key( ssl->handshake->psk_opaque );
-            ssl->handshake->psk_opaque_is_internal = 0;
-        }
-        ssl->handshake->psk_opaque = MBEDTLS_SVC_KEY_ID_INIT;
-    }
-#else
-    if( handshake->psk != NULL )
-    {
-        mbedtls_platform_zeroize( handshake->psk, handshake->psk_len );
-        mbedtls_free( handshake->psk );
-    }
-#endif /* MBEDTLS_USE_PSA_CRYPTO */
+    mbedtls_ssl_remove_psk( ssl );
 #endif
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C) && \