Fix to verify bitfields and aggregate signature.

In blockprocessing, `crosslink_committee` is not defined. This is a fix.

In Phase 0 this check is redundant since we've already asserted that the
custody bitfield is all zero, but it will matter in later phases.

specs/core/0_beacon-chain.md

@@ -1632,7 +1632,8 @@ For each `attestation` in `block.body.attestations`:
     assert attestation.custody_bitfield == b'\x00' * len(attestation.custody_bitfield)  # [TO BE REMOVED IN PHASE 1]
     assert attestation.aggregation_bitfield != b'\x00' * len(attestation.aggregation_bitfield)
 
-    for i in range(len(crosslink_committee)):
+    assert len(attestation.aggregation_bitfield) <= len(attestation.custody_bitfield)
+    for i in range(len(attestation.aggregation_bitfield)):
         if get_bitfield_bit(attestation.aggregation_bitfield, i) == 0b0:
             assert get_bitfield_bit(attestation.custody_bitfield, i) == 0b0