mailinfo: re-fix MIME multipart boundary parsing

mailinfo: re-fix MIME multipart boundary parsing

Recent changes to is_multipart_boundary() caused git-mailinfo to segfault.
The reason was after handling the end of the boundary the code tried to look
for another boundary.  Because the boundary list was empty, dereferencing
the pointer to the top of the boundary caused the program to go boom.

The fix is to check to see if the list is empty and if so go on its merry
way instead of looking for another boundary.

I also fixed a couple of increments and decrements that didn't look correct
relating to content_top.

The boundary test case was updated to catch future problems like this again.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>