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/20240902105656.1383858-4-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>