format_sanitized_subject: Don't trim past initial length of strbuf
format_sanitized_subject: Don't trim past initial length of strbuf

If the subject line is '...' the strbuf will be accessed before the
first dot is added; potentially changing the strbuf passed into the
function or accessing sb->buf[-1] if it was originally empty.

Reported-by: René Scharfe <5c721353370dda279bee7751cb8d3ff68ef77242@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git