Merge branch 'master' into interop

eth2/types/src/beacon_state.rs

@@ -398,6 +398,10 @@ impl<T: EthSpec> BeaconState<T> {
             .ok_or_else(|| Error::SlotOutOfBounds)?;
         let seed = self.get_seed(epoch, spec)?;
 
+        if first_committee.is_empty() {
+            return Err(Error::InsufficientValidators);
+        }
+
         let mut i = 0;
         Ok(loop {
             let candidate_index = first_committee[(epoch.as_usize() + i) % first_committee.len()];

eth2/types/src/test_utils/builders/testing_beacon_block_builder.rs

@@ -119,6 +119,10 @@ impl<T: EthSpec> TestingBeaconBlockBuilder<T> {
         // - The shard of the committee.
         let mut committees: Vec<(Slot, Vec<usize>, Vec<usize>, u64)> = vec![];
 
+        if slot < T::slots_per_epoch() {
+            panic!("slot is too low, will get stuck in loop")
+        }
+
         // Loop backwards through slots gathering each committee, until:
         //
         // - The slot is too old to be included in a block at this slot.