Rearrange seconds and nanoseconds in print-arista.c.

Rearrange seconds and nanoseconds in print-arista.c.

./print-arista.c:146:36: warning: implicit conversion loses integer
  precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t' (aka
  'unsigned int') [-Wshorten-64-to-32]

When the nanoseconds value is not less that one second, instead of
silently carrying the excess over to the seconds value flag the
timestamp as bogus, this way the decoder better shows what is on the
wire and the seconds value always fits into 32 bits.