Avoid c++11 nullptr and use 0 on null comparison

[filipecosta90]

Fixes #235

nullptr was introduced in c++11 and if your compiler does not support it will happen the error of #235 . This PR addresses it.

Reference: https://www.stroustrup.com/bs_faq2.html#null

[codecov]

Codecov Report

Merging #236 (1706538) into master (bec3471) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 1706538 differs from pull request most recent head d06d9ab. Consider uploading reports for the commit d06d9ab to get more accurate results

@@           Coverage Diff           @@
##           master     #236   +/-   ##
=======================================
  Coverage   56.41%   56.41%           
=======================================
  Files          21       21           
  Lines        4359     4359           
=======================================
  Hits         2459     2459           
  Misses       1900     1900           

Files Changed	Coverage Δ
memtier_benchmark.cpp	53.81% <100.00%> (ø)