Fix block & state queries prior to genesis (sigp#4761)

## Issue Addressed

Closes sigp#4751

## Proposed Changes

Prevent `state_root_at_slot` and `block_root_at_slot` from erroring out due to a call to `self.slot()?` that fails before genesis. This fixes pre-genesis queries for:

- block at slot 0
- block by genesis block root
- state at slot 0
- state by genesis state root
- state at `finalized` tag
- state at `justified` tag

Author: michaelsproul

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -801,10 +801,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     ///
     /// May return a database error.
     pub fn state_root_at_slot(&self, request_slot: Slot) -> Result<Option<Hash256>, Error> {
-        if request_slot > self.slot()? {
-            return Ok(None);
-        } else if request_slot == self.spec.genesis_slot {
+        if request_slot == self.spec.genesis_slot {
             return Ok(Some(self.genesis_state_root));
+        } else if request_slot > self.slot()? {
+            return Ok(None);
         }
 
         // Check limits w.r.t historic state bounds.
@@ -881,10 +881,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     ///
     /// May return a database error.
     fn block_root_at_slot_skips_none(&self, request_slot: Slot) -> Result<Option<Hash256>, Error> {
-        if request_slot > self.slot()? {
-            return Ok(None);
-        } else if request_slot == self.spec.genesis_slot {
+        if request_slot == self.spec.genesis_slot {
             return Ok(Some(self.genesis_block_root));
+        } else if request_slot > self.slot()? {
+            return Ok(None);
         }
 
         let prev_slot = request_slot.saturating_sub(1_u64);
@@ -944,10 +944,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     ///
     /// May return a database error.
     fn block_root_at_slot_skips_prev(&self, request_slot: Slot) -> Result<Option<Hash256>, Error> {
-        if request_slot > self.slot()? {
-            return Ok(None);
-        } else if request_slot == self.spec.genesis_slot {
+        if request_slot == self.spec.genesis_slot {
             return Ok(Some(self.genesis_block_root));
+        } else if request_slot > self.slot()? {
+            return Ok(None);
         }
 
         // Try an optimized path of reading the root directly from the head state.