Silently skip deposits with invalid proof in process_deposit

specs/core/0_beacon-chain.md

@@ -1252,13 +1252,14 @@ def process_deposit(state: BeaconState,
     Note that this function mutates ``state``.
     """
     # Validate the given `proof_of_possession`
-    assert validate_proof_of_possession(
+    valid_proof = validate_proof_of_possession(
         state,
         pubkey,
         proof_of_possession,
         withdrawal_credentials,
     )
 
+    if valid_proof:
         validator_pubkeys = [v.pubkey for v in state.validator_registry]
     
         if pubkey not in validator_pubkeys: