Replaced assert with BOOST_ASSERT.

[redboltz]

Applied *buffer buf and len checking assert.
Avoided needless code execution if buf is nullptr.

[codecov-commenter]

Codecov Report

Merging #898 into cpp_master will increase coverage by 0.02%.
The diff coverage is 78.94%.

@@              Coverage Diff               @@
##           cpp_master     #898      +/-   ##
==============================================
+ Coverage       86.43%   86.45%   +0.02%     
==============================================
  Files              72       72              
  Lines            5100     5108       +8     
==============================================
+ Hits             4408     4416       +8     
  Misses            692      692