fix(decoder): should reset crc16 when file's crc is 0

[muktihari]

• When d.fileHeader.CRC == 0x0000, it means the FIT file might be using legacy header (12 bytes) so there is no crc integrity for the file_header. However, the file_crc that will validate the integrity of the messages should be present. We should reset the d.crc16 before returning nil, otherwise the calculation of the messages' crc might mismatch since d.crc16 is still containing file_header's crc calculation. (We don't need reset d.crc16 on error)
• Clean up decoder's code

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (868984f) to head (d5e6eb8).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #221   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         3610      3607    -3     
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- Hits          3610      3607    -3     

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