A memleak bug of png2swf.

[ghost]

A memleak bug of png2swf.
poc1: https://drive.google.com/open?id=1MUuOab8EYCwhDUH9cC5DwSuF_slthBPd
asan1: https://drive.google.com/open?id=1h-w58olMDSpXohrNGDk-hLGEQjwhKDiO
poc2: https://drive.google.com/open?id=1QUpPmR9KQPWPsMVakzWhqgB8t0WQGW5f
asan2: https://drive.google.com/open?id=1jSFnQFLiiDPXVXZnYQZ7pRaK0g8PZJL7

https://github.com/matthiaskramm/swftools/blob/54657f9ba3dd4fa3e54c8f8c18f3def7a42d1f1c/lib/png.c#L517

In fuction png_load: when cmp with PLTE and tRNS, the data which is alloc is saved on palette/alphapalette, but not free, which can cause a memleak bug.

patch: free it