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.

notmuch-index-message.cc

@@ -486,6 +486,10 @@ gen_terms_body_str (Xapian::TermGenerator term_gen,
 	while (*next_line == '\n')
 	    next_line++;
 
+	/* Skip blank lines. */
+	if (line_end < line)
+	    continue;
+
 	/* Skip lines that are quotes. */
 	if (*line == '>')
 	    continue;