patch(1): Don't overrun line buffer in some cases

Patches like file.txt attached to PR 190195 with a final line formed
like ">(EOL)" could cause a copy past the end of the current line buffer. In the
case of PR 191641, this caused a duplicate line to be copied into the resulting
file.

Instead of running past the end, treat it as if it were a blank line.

PR: 191641
Reviewed by: cem, emaste, pfg
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D12609