Fix that CGImageDestinationCreateWithData 0 count arg will log a warning

[dreampiggy]

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• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

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This merge request fixes / refers to the following issues: #2738

Pull Request Description

See #2738. This log is from Image/IO, when the CGImageDestinationCreateWithData count arg is passed with 0 instead of 1.

2019-05-29 12:47:41.447767+0800 SDWebImage iOS Demo[33116:4568883] CGImageDestinationCreateWithData:3088: *** ERROR: CGImageDestinationCreateWithData: invalid capacity (0)

The warning seems from iOS 12+, but sadly, our Test scheme, and Example, disable the NSLog function totally. So this does not catch any attention during development. I'll suggest to remove those OS_ACTIVITY_MODE = disable in another PR.

[codecov]

Codecov Report

Merging #2739 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2739      +/-   ##
==========================================
- Coverage   84.04%   84.02%   -0.03%     
==========================================
  Files          56       56              
  Lines        6225     6227       +2     
==========================================
  Hits         5232     5232              
- Misses        993      995       +2

Flag	Coverage Δ
#ios	84.81% <100%> (-0.2%)	⬇️
#macos	83.03% <100%> (+0.37%)	⬆️

Impacted Files	Coverage Δ
SDWebImage/SDImageAPNGCoder.m	88.13% <100%> (+0.05%)	⬆️
SDWebImage/SDImageGIFCoder.m	89.43% <100%> (+0.04%)	⬆️
SDWebImage/SDWebImageManager.m	85.65% <0%> (-0.82%)	⬇️
SDWebImage/SDWebImageDownloader.m	80.83% <0%> (-0.3%)	⬇️
SDWebImage/SDWebImageDownloaderOperation.m	91.41% <0%> (+0.33%)	⬆️

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[kinarobin]

If animatedImage's imageFrame count is 0, Just as static image encode, This logic is OK.