bug: workerStack's binarySearch

[callthingsoff]

Avoid overflow when computing mid.

[panjf2000]

Well, I didn't want to overthink the possibility of this calculation overflowing the int type when we introduced this implemention of binary search, because if that's case, the amount of goroutines in ants would be too significant and it's meaningless to use a goroutine pool like ants. But I guess it's also OK for ants to be finicky here.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (9fdd99a) 92.45% compared to head (70eb6ad) 92.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #278      +/-   ##
==========================================
- Coverage   92.45%   92.44%   -0.01%     
==========================================
  Files           9        9              
  Lines         769      768       -1     
==========================================
- Hits          711      710       -1     
  Misses         44       44              
  Partials       14       14              

Flag	Coverage Δ
unittests	92.44% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
worker_stack.go	100.00% <100.00%> (ø)

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