Update CAO model loading to avoid "Exception: basic_ios::clear: iostream error"

[s-trinh]

lagadic/visp-images#26

Avoid exception when loading a CAO model with no new line at the end of the file:

Cannot get the number of circles. Defaulting to zero.
Exception: basic_ios::clear: iostream error

Remove pointers usage and add some sanity checks.

[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 6 lines in your changes missing coverage. Please review.

Project coverage is 53.84%. Comparing base (bc9a67e) to head (d195822).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
modules/tracker/mbt/src/vpMbTracker.cpp	83.78%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1529      +/-   ##
==========================================
+ Coverage   53.82%   53.84%   +0.01%     
==========================================
  Files         441      441              
  Lines       53859    53854       -5     
==========================================
+ Hits        28991    28995       +4     
+ Misses      24868    24859       -9     

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