Use SQLSRV_ASSERT for checking NULL ptrs

[yitam]

Use SQLSRV_ASSERT to ensure the subsequent code will not dereference any null pointers (Clang Analyzer reports)

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This change is 

[coveralls]

Coverage increased (+0.006%) to 74.706% when pulling 44d1bb3 on yitam:moreAsserts into 3585bf5 on Microsoft:dev.

[codecov-io]

Codecov Report

Merging #789 into dev will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #789      +/-   ##
==========================================
+ Coverage   80.06%   80.06%   +<.01%     
==========================================
  Files          25       25              
  Lines        7323     7325       +2     
==========================================
+ Hits         5863     5865       +2     
  Misses       1460     1460

Impacted Files	Coverage Δ
.../php-7.1.18-src/ext/sqlsrv/shared/core_results.cpp	74.22% <0%> (+0.04%)	⬆️
...-7.1.18-src/ext/pdo_sqlsrv/shared/core_results.cpp	59.96% <0%> (+0.07%)	⬆️

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