patch(1): Don't overrun line buffer if line is not completely malformed

[kevans]

Summary

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

[kevans]

Whoops, it's still possible to run out of memory there, and this looks a little
cleaner