Fix genKeyPair() generates not-fully-256bits private key

[sickworm]

I found a mistake which makes private key generated by genKeyPair() is not fully 256bits.

var key = sm2.genKeyPair();
console.log(key.toString());

// in current version will get:
// public: (cc9f54f037556f542e2683d7e6628dc9dcf9dd062ae1b9702aecf34e444649ed, 996ff71434f452362a5f941130d39ba25c9411119987d76801bf04b2eb8ea84),
// private: 00000000000d55f919276c8146c43ff52b449a7dac2043c94a2fb0bcf496aae0