Prevent bounds check bypass through overflow in PSK identity parsing

The check `if( *p + n > end )` in `ssl_parse_client_psk_identity` is
unsafe because `*p + n` might overflow, thus bypassing the check. As
`n` is a user-specified value up to 65K, this is relevant if the
library happens to be located in the last 65K of virtual memory.

This commit replaces the check by a safe version.

library/ssl_srv.c

@@ -3436,7 +3436,7 @@ static int ssl_parse_client_psk_identity( mbedtls_ssl_context *ssl, unsigned cha
     /*
      * Receive client pre-shared key identity name
      */
-    if( *p + 2 > end )
+    if( end - *p < 2 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client key exchange message" ) );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE );
@@ -3445,7 +3445,7 @@ static int ssl_parse_client_psk_identity( mbedtls_ssl_context *ssl, unsigned cha
     n = ( (*p)[0] << 8 ) | (*p)[1];
     *p += 2;
 
-    if( n < 1 || n > 65535 || *p + n > end )
+    if( n < 1 || n > 65535 || n > (size_t) ( end - *p ) )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client key exchange message" ) );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE );