boot: image: fixup zstd decompression buffer initialization typo

The code was mistakenly initializing the input buffer twice.

Tested to be working on BeagleBone by adjusting CONFIG_SYS_BOOTM_LEN to
64MiB (probably works with less) and preparing uImage with:

 cat arch/arm/boot/Image \
  | zstd --ultra -22 --zstd=windowLog=22 \
  > linux.bin.zst

 mkimage -A arm -T kernel uImage -C zstd -d linux.bin.zst \
  -a 0x80008000 -e 0x80008000

Without the windowLog restriction, bootm fails with a zstd decompression
error 7 (window too large), which I haven't troubleshooted.

There should be a bit more documentation on the feature...

Reviewed-by: Simon Glass <sjg@chromium.org>
Fixes: 458b30af image: Update image_decomp() to avoid ifdefs