Unchecked return value from bis.read() in MutationGroupEncoder#decodeMutation

```

private Mutation decodeMutation(ByteArrayInputStream bis) throws IOException {
Mutation.Op op
= Mutation.Op.values()[bis.read()];
if (op == Mutation.Op.DELETE) {
return decodeDelete(bis);

```

The return value from bis.read() should be checked before it is used to dereference values().

Imported from Jira [BEAM-3140](https://issues.apache.org/jira/browse/BEAM-3140). Original Jira may contain additional context.
Reported by: yuzhihong@gmail.com.

Start at MutationGroupEncoder#decodeMutation and inspect how bis.read() is used before Mutation.Op.values(). Confirm the return value is handled safely before lookup, then verify the relevant Beam tests pass and that truncated input no longer causes an unchecked lookup.

Written by the indexing model from the issue text.