make sure to call cancel func

[skaji]

Currently, go vet reports:

❯ go vet 2>&1 | grep mssql.go
./mssql.go:596:2: the cancel function is not used on all paths (possible context leak)
./mssql.go:616:5: this return statement may be reached without using the cancel var defined on line 596

This PR fixes these.

[codecov]

Codecov Report

Merging #544 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #544      +/-   ##
=========================================
+ Coverage   68.89%   68.9%   +0.01%     
=========================================
  Files          22      22              
  Lines        5066    5068       +2     
=========================================
+ Hits         3490    3492       +2     
  Misses       1370    1370              
  Partials      206     206

Impacted Files	Coverage Δ
mssql.go	82.77% <100%> (+0.05%)	⬆️

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