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:
345663c07a

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

usr.bin/indent/pr_comment.c

@@ -218,6 +218,7 @@ pr_comment(void)
 		dump_line();
 		return;
 	    }
+	    last_bl = NULL;
 	    if (ps.box_com || ps.last_nl) {	/* if this is a boxed comment,
 						 * we dont ignore the newline */
 		if (s_com == e_com) {