Avoid reading a byte just before our allocated buffer.

When looking for a trailing ':' to introduce a quotation we peek at
the last character before a newline. But for blank lines, that's not
where we want to look. And when the first line in our buffer is a
blank line, we're underrunning our buffer. The fix is easy---just
bail early on blank lines since they have no terms anyway.

Thanks to valgrind for pointing out this error.