bpf/proxy: stopExpandNPFixup only if runExpandNPFixup
bpf/proxy: stopExpandNPFixup only if runExpandNPFixup

If there was an error in Apply() before the fixer got started, there is
nothing to stop. Clean the stop channel after stop to avoid closing a
closed channel.

neiljerram/felix,neiljerram/felix,Metaswitch/calico,neiljerram/felix,neiljerram/felix,Metaswitch/calico