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: 458b30af66c image: Update image_decomp() to avoid ifdefs
diff --git a/boot/image.c b/boot/image.c
index 07fa2d3..121df0c 100644
--- a/boot/image.c
+++ b/boot/image.c
@@ -500,7 +500,7 @@
 			struct abuf in, out;
 
 			abuf_init_set(&in, image_buf, image_len);
-			abuf_init_set(&in, load_buf, unc_len);
+			abuf_init_set(&out, load_buf, unc_len);
 			ret = zstd_decompress(&in, &out);
 			if (ret >= 0) {
 				image_len = ret;