fix: key length check in decodeMerge

[ChALkeR]

Merge builds with 5 keys:

solana-web3.js/src/programs/stake.ts

Lines 884 to 894 in c85c8fd

	keys: [
	{pubkey: stakePubkey, isSigner: false, isWritable: true},
	{pubkey: sourceStakePubKey, isSigner: false, isWritable: true},
	{pubkey: SYSVAR_CLOCK_PUBKEY, isSigner: false, isWritable: false},
	{
	pubkey: SYSVAR_STAKE_HISTORY_PUBKEY,
	isSigner: false,
	isWritable: false,
	},
	{pubkey: authorizedPubkey, isSigner: true, isWritable: false},
	],

See also instruction.keys[4].pubkey access when throws when there are less than 5 keys