make sure chacha_crypt() can only be called after setting the IV

src/stream/chacha/chacha_crypt.c

@@ -57,9 +57,11 @@ int chacha_crypt(chacha_state *st, const unsigned char *in, unsigned long inlen,
    unsigned long i, j;
 
    if (inlen == 0) return CRYPT_OK; /* nothing to do */
-   LTC_ARGCHK(st  != NULL);
-   LTC_ARGCHK(in  != NULL);
-   LTC_ARGCHK(out != NULL);
+
+   LTC_ARGCHK(st        != NULL);
+   LTC_ARGCHK(in        != NULL);
+   LTC_ARGCHK(out       != NULL);
+   LTC_ARGCHK(st->ivlen != 0);
 
    if (st->ksleft > 0) {
       j = MIN(st->ksleft, inlen);