Update videoio.go

[buYoung]

Video Writer Ptr() add
This function can tell the nil state of VideoWriter

[codecov-commenter]

Codecov Report

Merging #700 into master will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
- Coverage   97.97%   97.88%   -0.09%     
==========================================
  Files          19       19              
  Lines        2220     2222       +2     
==========================================
  Hits         2175     2175              
- Misses         38       40       +2     
  Partials        7        7              

Impacted Files	Coverage Δ
videoio.go	86.07% <0.00%> (-2.24%)	⬇️

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[deadprogram]

Hello, @buYoung thanks for making a code contribution to GoCV. A couple of things:

• I switched the branch for this PR to dev, as mentioned here: https://github.com/hybridgroup/gocv/blob/master/CONTRIBUTING.md#how-to-use-our-github-repository
• Can you please add a "smoke test" as mentioned here? https://github.com/hybridgroup/gocv/blob/master/CONTRIBUTING.md#how-to-add-a-function-from-opencv-to-gocv

Thank you!