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)

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

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