RollingCharBuffer does not check all failed values.

### Description

The method code is as follows:

' public int get(int pos) throws IOException {
...
final int readCount = reader.read(buffer, nextWrite, toRead);
if (readCount == -1) {
end = true;
return -1;
}
'
When readCount fails, it can return 0. This value should aslo be checked.

https://github.com/apache/lucene/pull/13512

### Version and environment details

_No response_

Start with RollingCharBuffer.get(int pos) and inspect the reader.read call described in the issue. Review pull request 13512 for the related work; done means the method handles a zero readCount as well as -1 while preserving the existing end-of-input behavior.

Written by the indexing model from the issue text.