Accessing a negative index

I read following code and found next line is accessing a negative index. Is it true?
`response[i+1-_recv_delim_size]`
I assume following conditions.
i=0, recv_delim_size=2, recv_delimiter="\r\n"

```
bool ATParser::vrecv(const char *response, va_list args)

int i = 0;
int offset = 0;

while (response[i]) {
if (memcmp(&response[i+1-_recv_delim_size], _recv_delimiter, _recv_delim_size) == 0) {
i++;
```

Start in ATParser::vrecv at the response[i+1-_recv_delim_size] expression and inspect the loop with i=0, recv_delim_size=2, and the "\r\n" delimiter. Check whether the referenced memory is valid before memcmp and determine what behavior or test should confirm the result.

Written by the indexing model from the issue text.