go-mysqlstack: fix the panic close of closed channel #614

[zhyass]

[summary]
Fix the panic 'close of closed channel'.
[test case]
N/A
[patch codecov]
N/A

[codecov]

Codecov Report

Merging #616 into master will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #616   +/-   ##
======================================
- Coverage      89%     89%   -1%     
======================================
  Files         128     128           
  Lines       10206   10206           
======================================
- Hits         9153    9151    -2     
- Misses        711     712    +1     
- Partials      342     343    +1     

Impacted Files	Coverage Δ
src/backend/xacheck.go	83% <0%> (-2%)	⬇️
src/backend/txn.go	86% <0%> (-1%)	⬇️
src/executor/engine/merge_join.go	96% <0%> (+1%)	⬆️

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[BohuTANG]

LGTM.