buffers: protect against bad thread indices
buffers: protect against bad thread indices

There is a very rare bug in NAT processing that yeilds a thread
index of ~0.  When this happens, vlib_get_frame_queue_elt()
suffers a segfault and VPP quits.  Prevent an outright fault
by dropping the packet instead.

Type: fix
Signed-off-by: Jon Loeliger <2c20cf3f4bac0d9467d3fe2c932e4d1e1bc1f44c@netgate.com>
Change-Id: I48c7a268925bb821ea15e58db5d4bfb211c40c09

chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp