Update wrapper.go

[brucehuang2]

把判空提前，避免空指针错误

[codecov]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #12   +/-   ##
======================================
  Coverage    69.7%   69.7%           
======================================
  Files          11      11           
  Lines         723     723           
======================================
  Hits          504     504           
  Misses        180     180           
  Partials       39      39

Impacted Files	Coverage Δ
wrapper.go	70.4% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a01d2d...1facaad. Read the comment docs.

[fifsky]

thanks