net/vmxnet3: handle bad host framing

net/vmxnet3: handle bad host framing

The VMXNet3 protocol has a start-of-packet (SOP) and end-of-packet (EOP)
marker. If there was a bug where mbuf arrived without SOP the code that
chains the mbuf would dereference a null pointer.
Also, record any mbuf's dropped in statistics.

Although did the initial code no longer have access to VMware.
Compile tested only!

Coverity issue: 124563
Fixes: 8ee787ce80a8 ("vmxnet3: remove asserts that confuse coverity")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Yong Wang <yongwang@vmware.com>