common: replace NULL with nullptr

[mbiesek]

Intention of this PR is to replace NULL with nullptr.
Test/examples are compiled with -std=c++11, which means nullptr value is supported by these module.
I could prepare additional PR for benchmark module with similar changes and additional for makefile to change the std to c++11, but I don't know if it is possible, because of:
485a455

In any case any feedback will be appreciated.

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This change is 

[codecov-io]

Codecov Report

Merging #2836 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2836      +/-   ##
==========================================
+ Coverage   77.21%   77.21%   +<.01%     
==========================================
  Files         175      175              
  Lines       25174    25175       +1     
==========================================
+ Hits        19437    19438       +1     
  Misses       5737     5737

[marcinslusarz]

I'm not sure what's up with pmembench. @krzycz?

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Reviewed 11 of 11 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[krzycz]

I can't remember what was the actual problem, but since the examples are compiled with -std=c++11 and it works with Coverity, then we can probably do the same for pmembench.

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Reviewed 11 of 11 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable