feat: verifies input range in the computeRoot to avoid panic

[staheri14]

Overview

Closes https://github.com/celestiaorg/nmt/issues/137

Checklist

• [x] New and updated code has appropriate documentation
• [x] New and updated code has new and/or updated testing
• [x] Required CI checks are passing
• [x] Visual proof for any user facing features like CLI or documentation updates
• [x] Linked issues closed with keywords

[codecov[bot]]

Codecov Report

Merging #186 (eecd425) into master (1262b7f) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   95.69%   95.70%   +0.01%     
==========================================
  Files           5        5              
  Lines         557      559       +2     
==========================================
+ Hits          533      535       +2     
  Misses         14       14              
  Partials       10       10              

Impacted Files	Coverage Δ
nmt.go	98.01% <100.00%> (+0.01%)	:arrow_up: