infohash: always write end of string terminator

include/opendht/infohash.h

@@ -384,6 +384,7 @@ Hash<N>::to_c_str() const
         const auto& m = hex_map[data_[i]];
         *((uint16_t*)b) = *((uint16_t*)&m);
     }
+    buf[N*2] = '\0';
     return buf.data();
 }