fix decoding on empty shards and when less than $k$ (dragoon/komodo!110)

makes sure
- "inbreeding" experiment quits when there are less than $k$ shards
- `fec::decode` returns `KomodoError::TooFewShards` when no shards are provided

examples/inbreeding/mod.rs

@@ -158,6 +158,10 @@ where
     pb.set_style(sty.clone());
     pb.set_message("main");
     for t in 0..=max_t {
+        if shards.len() < k {
+            break;
+        }
+
         if measurement_schedule(t) {
             let inbreeding = measure_inbreeding(&shards, k, nb_measurements, &mp, &sty, rng);
             println!("{}", inbreeding);

src/fec.rs

@@ -160,6 +160,10 @@ pub fn encode<F: PrimeField>(
 /// > - if there are too few shards
 /// > - if there are linear dependencies between shards
 pub fn decode<F: PrimeField>(shards: Vec<Shard<F>>) -> Result<Vec<u8>, KomodoError> {
+    if shards.is_empty() {
+        return Err(KomodoError::TooFewShards(0, 0));
+    }
+
     let k = shards[0].k;
     let np = shards.len();