[pktbuf] Fix NULL deref when the freelist is empty
[pktbuf] Fix NULL deref when the freelist is empty

nvll/lk,skabet/lk,travisg/lk,skabet/lk,ErikCorryGoogle/lk,srodrig1/lk,hollanderic/lkstuff,M1cha/lk,srodrig1/lk,GCrean/lk,srodrig1/lk,hollanderic/lkstuff,M1cha/lk,ErikCorryGoogle/lk,ErikCorryGoogle/lk,sndnvaps/lk,nvll/lk,hollanderic/lkstuff,grub4android/lk,sndnvaps/lk,grub4android/lk,minglun-tsai/lk,grub4android/lk,minglun-tsai/lk,skabet/lk,GCrean/lk,minglun-tsai/lk,skabet/lk,skabet/lk,srodrig1/lk,littlekernel/lk,nvll/lk,ErikCorryGoogle/lk,travisg/lk,nvll/lk,hollanderic/lkstuff,sndnvaps/lk,GCrean/lk,M1cha/lk,littlekernel/lk,GCrean/lk,M1cha/lk,littlekernel/lk,grub4android/lk,nvll/lk,srodrig1/lk,littlekernel/lk,minglun-tsai/lk,travisg/lk,GCrean/lk,sndnvaps/lk,grub4android/lk,sndnvaps/lk,travisg/lk,M1cha/lk