ref(decoding): simplify conditional branch

[JensRantil]

...by moving out common code.

[codecov]

Codecov Report

Merging #355 into master will increase coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
+ Coverage   81.48%   81.80%   +0.31%     
==========================================
  Files          41       41              
  Lines        5013     5006       -7     
==========================================
+ Hits         4085     4095      +10     
+ Misses        807      793      -14     
+ Partials      121      118       -3     

Impacted Files	Coverage Δ
reflect_struct_decoder.go	48.46% <100.00%> (+1.38%)	⬆️
iter_float.go	88.41% <0.00%> (-0.20%)	⬇️
reflect_marshaler.go	80.64% <0.00%> (-0.16%)	⬇️
reflect_extension.go	82.98% <0.00%> (+0.82%)	⬆️
stream.go	90.00% <0.00%> (+4.00%)	⬆️

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