Replaced mbedtls_ssl_safer_memcmp with mbedtls_platform_memcmp

Saves some bytes and mbedtls_platform_memcmp is a bit safer for side channel
attacks.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index c9b03f5..b41ba32 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -160,7 +160,7 @@
         /* Check verify-data in constant-time. The length OTOH is no secret */
         if( len    != 1 + ssl->verify_data_len ||
             buf[0] !=     ssl->verify_data_len ||
-            mbedtls_ssl_safer_memcmp( buf + 1, ssl->peer_verify_data,
+            mbedtls_platform_memcmp( buf + 1, ssl->peer_verify_data,
                           ssl->verify_data_len ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching renegotiation info" ) );
@@ -4089,7 +4089,7 @@
         /* Identity is not a big secret since clients send it in the clear,
          * but treat it carefully anyway, just in case */
         if( n != ssl->conf->psk_identity_len ||
-            mbedtls_ssl_safer_memcmp( ssl->conf->psk_identity, *p, n ) != 0 )
+            mbedtls_platform_memcmp( ssl->conf->psk_identity, *p, n ) != 0 )
         {
             ret = MBEDTLS_ERR_SSL_UNKNOWN_IDENTITY;
         }