span: crash in span_mirror [VPP-1254]

span: crash in span_mirror [VPP-1254]

It is possible for span-input to get call with sw_if_index which is greater than
sm->interfaces and crashes in span_mirror () in the following line

  span_interface_t *si0 = vec_elt_at_index (sm->interfaces, sw_if_index0);

For example, span-input mirrors a main interface as source, it may actually get
call for traffic coming in from the subinterface and crashes.

The fix is simply to check if sw_if_index >= vec_len (sm->interfaces) and
punt if it is.

Change-Id: I8312eb321d638518e14ba2326fffd1a7919646ca
Signed-off-by: Steven <sluong@cisco.com>