output_frag: don't double free() if a compressed payload doesn't need to be fragmented
output_frag: don't double free() if a compressed payload doesn't need to be fragmented

it was possible for a double free() to occur in _output_frag_write() if
compression was enabled and the payload, after compression, was small
enough to fit in a normal NMSG container without fragmentation.

hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg