coverity 1462573 Dereference after null check

Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11651)

crypto/evp/pmeth_gn.c

@@ -93,7 +93,7 @@ static int gen_init(EVP_PKEY_CTX *ctx, int operation)
 #endif
 
  end:
-    if (ret <= 0)
+    if (ret <= 0 && ctx != NULL)
         ctx->operation = EVP_PKEY_OP_UNDEFINED;
     return ret;