lz4: Cherrypick fix for CVE-2021-3520

There should be no risk of us accidentally hitting this since
we'd need maliciously malformed data to wind up in the pipeline,
or a very unfortunate random bit flip at exactly the right moment.
Still since we can handle it we should.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam Moss <c@yotes.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12947

module/zfs/lz4.c

@@ -541,7 +541,7 @@ LZ4_decompress_generic(
                  const size_t dictSize         /* note : = 0 if noDict */
                  )
 {
-    if (src == NULL) { return -1; }
+    if ((src == NULL) || (outputSize < 0)) { return -1; }
 
     {   const BYTE* ip = (const BYTE*) src;
         const BYTE* const iend = ip + srcSize;