mbox: Fix get_body for multipart email
mbox: Fix get_body for multipart email

get_payload is documented to return None if decode is True for multipart
message. In this case, we can only decode the child payload object.

Signed-off-by: Fam Zheng <df9fd617995bad07eac70e27aa3fe5069270e3bd@redhat.com>

pvital/patchew,pvital/patchew,pvital/patchew,pvital/patchew