Fix: taskPool will execute task when session was nil

[Lvnszn]

What this PR does:
First task was be added into task pool, then session will be set to nil with connection interrupted and task was executed by goroutine will lead to panic.
Which issue(s) this PR fixes:

Fixes #88

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #91 (d25d2a8) into master (76136d0) will decrease coverage by 0.05%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   69.14%   69.09%   -0.06%     
==========================================
  Files           8        8              
  Lines        1439     1443       +4     
==========================================
+ Hits          995      997       +2     
- Misses        348      349       +1     
- Partials       96       97       +1     

Impacted Files	Coverage Δ
session.go	69.66% <50.00%> (-0.17%)	⬇️

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