PROV: Avoid NULL dereference in SHA3 dup call.

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10487)

providers/implementations/digests/sha3_prov.c

@@ -241,7 +241,8 @@ static void *keccak_dupctx(void *ctx)
     KECCAK1600_CTX *in = (KECCAK1600_CTX *)ctx;
     KECCAK1600_CTX *ret = OPENSSL_malloc(sizeof(*ret));
 
-    *ret = *in;
+    if (ret != NULL)
+        *ret = *in;
     return ret;
 }