Avoid an implicit clone of the Orchard bundle in ContextualCheckTransaction.

C++ `auto`, as opposed to `auto&`, does not infer reference types (except when it does:
https://blog.petrzemek.net/2017/12/08/when-auto-seemingly-deduces-a-reference-in-cpp/
but that doesn't apply here). The inferred type of `orchard_bundle` was therefore
`const OrchardBundle`. This made the assignment a call to `OrchardBundle::operator=`
which calls `orchard_bundle_clone`. A reference assignment is sufficient, because `tx`
is in scope throughout `ContextualCheckTransaction`.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

src/main.cpp

@@ -861,7 +861,7 @@ bool ContextualCheckTransaction(
     assert(!nu5Active || canopyActive);         // NU5 cannot be active unless Canopy is
     assert(!futureActive || nu5Active);         // ZFUTURE must include consensus rules for all supported network upgrades.
 
-    auto const orchard_bundle = tx.GetOrchardBundle();
+    auto& orchard_bundle = tx.GetOrchardBundle();
 
     // Rules that apply only to Sprout
     if (beforeOverwinter) {