perform boundary check before searching to prevent security vulnerabilities

src/str.cpp

@@ -266,6 +266,7 @@ int utf8len(unsigned char c, bool suppress){
     }
 
     int Str::index(const Str& sub, int start) const {
+        if (start < 0 || start >= this->u8_length()) return -1;
         auto p = std::search(data + start, data + size, sub.data, sub.data + sub.size);
         if(p == data + size) return -1;
         return p - data;