fib: FIB crash removing labelled route (VPP-1818)

fib: FIB crash removing labelled route (VPP-1818)

Type: fix

The crash occured trying to retreive a NULL path list to walk the path
extensions. A walk shoul not be required, because there should be no
extensins, since all paths are removed. The problem is that when the
paths were added, they were not sorted, hence neither were the
extensions and when they were updated, duplicate extensions were added,
and hence a path removal did not remove them all.
Fix is to make sure paths are sorted.

Change-Id: I069d937de8e7bc8aae3d92f588db4daff727d863
Signed-off-by: Neale Ranns <nranns@cisco.com>