lib: zstd: fix null-deref in ZSTD_createCDict_advanced2()

lib: zstd: fix null-deref in ZSTD_createCDict_advanced2()

ZSTD_createCDict_advanced2() must ensure that
ZSTD_createCDict_advanced_internal() has successfully
allocated cdict.  customMalloc() may be called under
low memory condition and may be unable to allocate
workspace for cdict.

Link: https://lkml.kernel.org/r/20240712051850.484318-4-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>