Missing whistleblower index check

[paulhauner]

Description

We fail to ensure that the whistleblower_index is in bounds:

lighthouse/consensus/state_processing/src/common/slash_validator.rs

Lines 49 to 53 in 8a16548

	increase_balance(
	state,
	whistleblower_index,
	whistleblower_reward.safe_sub(proposer_reward)?,
	)?;

This is not exploitable in this version of the spec, however we should still fix it.