mgm: decrypt only after tag verification

[newpavlov]

It results in two-pass decryption which may be a bit less performant than the current approach, but should be a bit more secure in some contexts.

[codecov-commenter]

Codecov Report

Merging #356 (991a3df) into master (5983bc0) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #356      +/-   ##
==========================================
+ Coverage   89.19%   89.24%   +0.05%     
==========================================
  Files          39       39              
  Lines        1897     1888       -9     
==========================================
- Hits         1692     1685       -7     
+ Misses        205      203       -2     

Impacted Files	Coverage Δ
mgm/src/lib.rs	100.00% <100.00%> (+1.98%)	⬆️

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