fix another boundary case and hopefully improve performance

fix another boundary case and hopefully improve performance

The fix: the if we found ourselves at a charstart with only one
character to read, readsize would be zero, handle that correctly.

Performance: originally I read just the first byte of the next
character, which meant as many extra read calls as there are
characters left to read after the initial read.  So O(Nleft) reads
where Nleft is the number of characters left to read after the initial
read.

Now read as many bytes as there are characters left to read, which
should mean the number of reads comes down to O(log(Nleft**2)) I think
(but don't ask me to justify that.)