fix: mesgdef should clear array pool directly

[muktihari]

• Since we use append on field array pool, it may grow more than the size of the field array pool. New underlying array will be created, leave behind field array pool data untouched when doing clear.
• There is also bug on clearing field array pool on creating unknown messages, if we end up creating new slice, clear(arr[:len(unknownFields)]) may panic as the len of unknownFields is more than arr.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b85b163) to head (8f7976e).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #479   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           42        42           
  Lines         3810      3810           
=========================================
  Hits          3810      3810           

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