Always add a suitable error if we fail to decode

We're always supposed to add the fallback "unsupported" error if we don't
have anything better. However in some cases this wasn't happening because
we were incorrectly setting "flag_construct_called" - even though the
construct function had failed.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #21603)

(cherry picked from commit 564e5b7)

crypto/encode_decode/decoder_lib.c

@@ -750,10 +750,11 @@ static int decoder_process(const OSSL_PARAM params[], void *arg)
                            (void *)new_data.ctx, LEVEL, rv);
             } OSSL_TRACE_END(DECODER);
 
-            data->flag_construct_called = 1;
             ok = (rv > 0);
-            if (ok)
+            if (ok) {
+                data->flag_construct_called = 1;
                 goto end;
+            }
         }
 
         /* The constructor didn't return success */