Stop Unmarshaling if Offsets Ended

[rauljordan]

This fixes a panic scenario in which we would attempt to marshal and unmarshal data that looks as follows:

type block struct {
	Slot uint64
	Body *blockBody
}

b := &block{
        Slot: 55,
}

We should detect if we have finished reading all variable offset values and stop unmarshaling instead of attempting to continue. A regression test was added to prevent this issue from popping up again.

[prestonvanloon]

LGTM

[codecov]

Codecov Report

Merging #69 into master will increase coverage by 0.31%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   75.57%   75.88%   +0.31%     
==========================================
  Files          12       12              
  Lines        1138     1132       -6     
==========================================
- Hits          860      859       -1     
+ Misses        159      155       -4     
+ Partials      119      118       -1

Impacted Files	Coverage Δ
types/struct.go	74.54% <100%> (+0.2%)	⬆️
ssz.go	53.62% <0%> (-2.9%)	⬇️
types/determine_size.go	83.19% <0%> (+2.52%)	⬆️
deep_equal.go	64.1% <0%> (+2.56%)	⬆️

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