Fix the case when SDWebImageFirstFrameOnly hit memory cache, the animated image still been provided

[dreampiggy]

See SDWebImage/SDWebImage#2725

[codecov]

Codecov Report

Merging #2 into master will decrease coverage by 0.06%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage   65.28%   65.22%   -0.07%     
==========================================
  Files           7        7              
  Lines         484      486       +2     
==========================================
+ Hits          316      317       +1     
- Misses        168      169       +1

Flag	Coverage Δ
#iOS	65.22% <50%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
...lasses/YYCache/YYCacheBridge/YYCache+SDAdditions.m	51.11% <50%> (-0.01%)	⬇️

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