tools: patman: Generate cover letter correctly when 'END' is missing

If 'END' is missing in a 'Cover-letter' section, and that section
happens to show up at the very end of the commit message, and the
commit is the last commit of the series, patman fails to generate
cover letter for us. Handle this in CloseCommit of patchstream.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>