[MAC] Fix LogicalLoraChannelHelper Object double destruction

[non-det-alle]

Bug explanation: SetLogicalLoraChannelHelper() assigned a plain LogicalLoraChannelHelper Object (i.e., not a Ptr) to LorawanMac's member. Objects do not have a dedicated assignment operator implementation so their members are copied without modifications. Objects internally store a pointer to a buffer of aggregated objects. When the copy-assigned function parameter Object goes out of scope, its destructor is called, clearing the buffer of aggregated objects stored inside. The same buffer address was also stored in the LorawanMac's member by the copy assignment. At LorawanMac destruction time, this causes a second attempt at clearing the buffer, accessing already unallocated memory.

Note: The examples do not end up in a segmentation fault because currently ref counting pointers are not correctly released at the end of the simulation, effectively causing a memory leak. In the future we must implement DoDispose.

Note2: Found this while trying to implement MAC layer tests for coverage of #179.

[codecov]

Codecov Report

Attention: Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.

Project coverage is 77.66%. Comparing base (82ea58a) to head (34f226c).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
model/end-device-lorawan-mac.cc	61.53%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #180   +/-   ##
========================================
  Coverage    77.66%   77.66%           
========================================
  Files           66       66           
  Lines         6831     6831           
========================================
  Hits          5305     5305           
  Misses        1526     1526           

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