sd-journal: properly convert object->size on big endian

mmap code crashes when attempting to map an object of zero size.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=758392
https://bugs.freedesktop.org/show_bug.cgi?id=82894
sd-journal: verify that object start with the field name

If the journal is corrupted, we might return an object that does
not start with the expected field name and/or is shorter than it
should.