kdf: avoid NULL dereference on malloc failure in sshkdf

Fixes #18009

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from #18011)

providers/implementations/kdfs/sshkdf.c

@@ -60,7 +60,8 @@ static void *kdf_sshkdf_new(void *provctx)
 
     if ((ctx = OPENSSL_zalloc(sizeof(*ctx))) == NULL)
         ERR_raise(ERR_LIB_PROV, ERR_R_MALLOC_FAILURE);
-    ctx->provctx = provctx;
+    else
+        ctx->provctx = provctx;
     return ctx;
 }