dont't panic when marshal from nil or unmarshal to nil interface or pointer

[AllenX2018]

This pr avoid go-toml panic when calling Marshal(v interface{}) or Unmarshal(data []byte, v interface{}) with nil or nil-pointer as v, just return an error in these cases.

[codecov]

Codecov Report

Merging #353 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   94.92%   94.94%   +0.01%     
==========================================
  Files          10       10              
  Lines        2087     2095       +8     
==========================================
+ Hits         1981     1989       +8     
  Misses         64       64              
  Partials       42       42              

Impacted Files	Coverage Δ
marshal.go	94.15% <100.00%> (+0.09%)	⬆️

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[pelletier]

Good idea, thank you for taking care of this!