cmd: ximg: handle Z_BUF_ERROR explicitly in GZIP decompression

cmd: ximg: handle Z_BUF_ERROR explicitly in GZIP decompression

When decompressing GZIP-compressed image parts via the `imxtract` command,
explicitly handle the `Z_BUF_ERROR` return value from `gunzip()` to provide
a clearer diagnostic. This error typically indicates that the destination
buffer is too small to hold the uncompressed data.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
[trini: Rework to indent the whole case with { } due to not using the
C23 extension]
Signed-off-by: Tom Rini <trini@konsulko.com>