net: nbuf: Fix double free in net_nbuf_compact()

If we are in the end of the fragment chain, then we can just
bail out as there is nothing more to do. There will be a
double free if we continue as the last entry is already
removed at this point.

Change-Id: I0f9782b408244d283dc7e3e087359dd00bada7a9
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>