Add regression test found by fuzzer

[prestonvanloon]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

This prevents a panic out of range issue.

Which issues(s) does this PR fix?

N/A

Other notes for review

This bug was discovered by taking a randomized beacon state and advancing empty state transitions until the next epoch start.

Here's the beacon state that generated this input: crash-2994ca80dfcefc8e626f965c109dcc762710427e.zip

Reproduction steps:

• UnmarshalSSZ

	nextEpoch := helpers.SlotToEpoch(s.Slot())+1
	if _, err := stateutil.ProcessSlots(context.Background(), s, helpers.StartSlot(nextEpoch)); err != nil {
		panic(err)
	}

[codecov]

Codecov Report

Merging #6628 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6628      +/-   ##
==========================================
- Coverage   61.81%   61.80%   -0.01%     
==========================================
  Files         377      374       -3     
  Lines       29184    29281      +97     
==========================================
+ Hits        18039    18097      +58     
- Misses       8669     8697      +28     
- Partials     2476     2487      +11