#47 crash due to invalid access in simplestring_addn function

[Luan Nguyen]

Hi Team,

In function simplestring_addn:

void simplestring_addn(simplestring target, const char source, int add_len) {
...
...

 if(target->str) {
     if(add_len) {
        memcpy(target->str + target->len, source, add_len);
     }
     target->len += add_len;
     target->str[target->len] = 0; /* null terminate */  // (1) crash here
  }

}
}

target->len is declared as int, so it could has negative (when string length is larger than 2Gb) value then an sign extension operation could cause target->len has a very big value and crash in the access to target->str[target->len].

Please issue a fix. Thanks.