Simplified get_field_as_string and made it more robust

[yitam]

Use the returned length from SQLGetData as an estimate and pad the buffer to avoid string truncation warning that might result with auto translation behind the scene.
See related issue #1261

[codecov]

Codecov Report

Merging #1265 (e4a2505) into dev (a14cb70) will increase coverage by 0.08%.
The diff coverage is n/a.

❗ Current head e4a2505 differs from pull request most recent head edaed5b. Consider uploading reports for the commit edaed5b to get more accurate results

@@            Coverage Diff             @@
##              dev    #1265      +/-   ##
==========================================
+ Coverage   83.43%   83.52%   +0.08%     
==========================================
  Files          22       22              
  Lines        7812     7780      -32     
==========================================
- Hits         6518     6498      -20     
+ Misses       1294     1282      -12     

Impacted Files	Coverage Δ
.../php-8.0.7-src/ext/pdo_sqlsrv/shared/core_stmt.cpp	83.70% <0.00%> (+0.26%)	⬆️
.../x86/php-8.0.7-src/ext/sqlsrv/shared/core_stmt.cpp	85.31% <0.00%> (+0.28%)	⬆️