Better tests for types package

[gdbelvin]

Add checks for decoding to a nil ptr and invalid type lengths.

Cherry picked the orthogonal changes to types out of #1037.

[codecov-io]

Codecov Report

Merging #1070 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1070      +/-   ##
=========================================
+ Coverage   62.08%   62.1%   +0.01%     
=========================================
  Files         104     104              
  Lines        8553    8557       +4     
=========================================
+ Hits         5310    5314       +4     
  Misses       2703    2703              
  Partials      540     540

Impacted Files	Coverage Δ
types/logroot.go	93.33% <100%> (+0.47%)	⬆️
types/maproot.go	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 734f47a...fc3ce73. Read the comment docs.