Made the code less ambiguous, removed compiler warning, no functional change

ntru_crypto_ntru_encrypt.c: In function ‘ntru_crypto_ntru_decrypt’:
ntru_crypto_ntru_encrypt.c:586:28: warning: operation on ‘*(ringel_buf2 + (sizetype)((unsigned int)i * 2u))’ may be undefined [-Wsequence-point]
             ringel_buf2[i] = --ringel_buf2[i] & mod_q_mask;
                            ^