Extend output buffer sizes with SQL decimals or numerics

[yitam]

When the sql type is strictly defined (e.g. decimal(1,0)), the column size is only 1. Since decimals and numerics are treated as strings, the initial values of the output params with trailing zeros are dropped in convert_to_string() as well.

Thus the output buffer sizes are not enough for negative numbers (with a negative sign) and/or decimals.

See the similar issue #705

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This change is 

[coveralls]

Coverage increased (+0.03%) to 73.185% when pulling 64e2241 on yitam:vso2913 into 196fe49 on Microsoft:dev.

[codecov-io]

Codecov Report

Merging #775 into dev will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #775      +/-   ##
==========================================
+ Coverage   78.92%   78.96%   +0.03%     
==========================================
  Files          25       25              
  Lines        7213     7215       +2     
==========================================
+ Hits         5693     5697       +4     
+ Misses       1520     1518       -2

Impacted Files	Coverage Δ
...php-7.1.17-src/ext/pdo_sqlsrv/shared/core_stmt.cpp	66.34% <0%> (+0.03%)	⬆️
...x86/php-7.1.17-src/ext/sqlsrv/shared/core_stmt.cpp	83.44% <0%> (+0.23%)	⬆️

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