l2_learning: Don't try to drop nonexistent buffers
l2_learning: Don't try to drop nonexistent buffers

(Resolves issue #9)

l2_learning's drop() function is sometimes called simply to
make the switch consume a buffer by sending a packet_out. BUT,
this is called regardless of whether the switch buffered the
packet or not (i.e., if buffer_id is -1 or not). If there's no
buffer, there's no point in doing this to begin with AND the
switch will probably return an error, so in this case we just
shouldn't do it. :)

MurphyMc/pox,PrincetonUniversity/pox,noxrepo/pox,adusia/pox,denovogroup/pox,noxrepo/pox,chenyuntc/pox,denovogroup/pox,andiwundsam/_of_normalize,denovogroup/pox,kpengboy/pox-exercise,kavitshah8/SDNDeveloper,kulawczukmarcin/mypox,xAKLx/pox,kulawczukmarcin/mypox,andiwundsam/_of_normalize,kulawczukmarcin/mypox,kulawczukmarcin/mypox,chenyuntc/pox,diogommartins/pox,carlye566/IoT-POX,carlye566/IoT-POX,pthien92/sdn,xAKLx/pox,PrincetonUniversity/pox,waltznetworks/pox,noxrepo/pox,MurphyMc/pox,VamsikrishnaNallabothu/pox,xAKLx/pox,pthien92/sdn,kulawczukmarcin/mypox,adusia/pox,carlye566/IoT-POX,chenyuntc/pox,jacobq/csci5221-viro-project,kpengboy/pox-exercise,chenyuntc/pox,pthien92/sdn,waltznetworks/pox,denovogroup/pox,VamsikrishnaNallabothu/pox,pthien92/sdn,jacobq/csci5221-viro-project,waltznetworks/pox,noxrepo/pox,VamsikrishnaNallabothu/pox,denovogroup/pox,andiwundsam/_of_normalize,jacobq/csci5221-viro-project,waltznetworks/pox,pthien92/sdn,diogommartins/pox,carlye566/IoT-POX,VamsikrishnaNallabothu/pox,adusia/pox,adusia/pox,kpengboy/pox-exercise,diogommartins/pox,chenyuntc/pox,kpengboy/pox-exercise,carlye566/IoT-POX,kavitshah8/SDNDeveloper,andiwundsam/_of_normalize,kpengboy/pox-exercise,adusia/pox,kavitshah8/SDNDeveloper,diogommartins/pox,kavitshah8/SDNDeveloper,PrincetonUniversity/pox,waltznetworks/pox,MurphyMc/pox,jacobq/csci5221-viro-project,PrincetonUniversity/pox,MurphyMc/pox,diogommartins/pox,xAKLx/pox,PrincetonUniversity/pox,MurphyMc/pox,jacobq/csci5221-viro-project,VamsikrishnaNallabothu/pox,xAKLx/pox