Fix a `bytearray index out of range` error while reading a string.

[pufit]

Sometimes, a data chunk we are trying to read from CH can exceed a buffer size. If it happens, the data will be split into two parts. In some cases it can happen during the read_varint, so we must fetch the next buffered chunk before finishing the read.

Closes

• https://github.com/long2ice/asynch/issues/69
• https://github.com/long2ice/asynch/issues/20

[itssimon]

Just hit this as well in production.

@long2ice This would be another good one to merge and release to make asynch more stable!

[stankudrow]

@pufit , could you possibly provide a detailed example to reproduce the problem? @itssimon , does this change solve the problem? If so, @long2ice , wouldn't you mind to merge the PR?

[long2ice]

Thanks!