Fix the encoding leak when input CGImage is not RGBA8888 or RGB888

[dreampiggy]

See Apple's documentation about vImageBuffer_InitWithCGImage

You're responsible for releasing the memory that buf->data points to back to the system using free().

https://developer.apple.com/documentation/accelerate/1399118-vimagebuffer_initwithcgimage?language=objc

[codecov]

Codecov Report

Merging #54 (44b1aae) into master (7d68ede) will not change coverage.
The diff coverage is 37.50%.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   77.49%   77.49%           
=======================================
  Files           2        2           
  Lines         813      813           
=======================================
  Hits          630      630           
  Misses        183      183           

Flag	Coverage Δ
ios	77.49% <37.50%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
SDWebImageWebPCoder/Classes/SDImageWebPCoder.m	77.37% <37.50%> (ø)

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