indent(1): Fix wrapping of some lines in comments.

After a blank line was printed (to separate paragraphs in comments), the
next line was sometimes wrapped to the column at which the previous
non-empty line ended. The fix is to reset the last blank pointer (last_bl)
on newline.

References:
https://github.com/pstef/freebsd_indent/commit/345663c07af0758fd10433bde14722dfd900f85c

Differential Revision: https://reviews.freebsd.org/D6966 (Partial)
Submitted by: Piotr Stefaniak