fix: BufReader should not panic when used after cancellation

[losfair]

If a BufReader::fill_buf() call is cancelled when the internal buffer is held by .read(), re-allocate the buffer when used in the future.

[CLAassistant]

All committers have signed the CLA.

[losfair]

Closing this - simply re-allocating the buffer is incorrect because cancellation is asynchronous and this can result in lost data.

The error message could be improved though...