This isn't a straightforward fix, unfortunately. The existing GetLine()
implementation didn't deal well with input that's incrementally produced
where individually read chunks wouldn't end with the separator.
The prior implementation increased the buffer each time it failed to find
a separator in the current buffer, but then also ended up not searching the
full new buffer size for the terminator, doing that endlessly.
This change reworks the Raw reader to rely only on bufpos for reading
and searching purposes and skip reallocation if the buffer size if it
wasn't actually exhausted.
Closes#3957
