lib: decompress_unzstd: Limit output size

The zstd decompression code, as it is right now, will most likely fail
on 32-bit systems, as the default output buffer size causes the buffer's
end address to overflow.

Address this issue by setting a sane default to the default output size,
with a value that won't overflow the buffer's end address.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Nick Terrell <terrelln@fb.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>