fix 32bit machine bug

[sloanyyc]

(((n - 1 - i) >> 31) & '=') | ... -----> (((n - 1 - i) >> 23) & '=') | ... 32bit core -1 >> 31 0x00000001 64bit core -1 >> 31 0x00000001ffffffff

32bit core -1 >> 23 0x000001ff 64bit core -1 >> 23 0x000001ffffffffff base64 output limit to 2^23