refactor: Work around GCC 9 -Wredundant-move warning

Use std::move workaround for unique_ptr, for when the C++ compiler lacks
a fix for this issue:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579
Do this in a way that avoids a GCC 9 `-Wredundant-move` warning.

Author: ryanofsky

src/interfaces/chain.cpp

@@ -236,11 +236,10 @@ class ChainImpl : public Chain
     explicit ChainImpl(NodeContext& node) : m_node(node) {}
     std::unique_ptr<Chain::Lock> lock(bool try_lock) override
     {
-        auto result = MakeUnique<LockImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
-        if (try_lock && result && !*result) return {};
-        // std::move necessary on some compilers due to conversion from
-        // LockImpl to Lock pointer
-        return std::move(result);
+        auto lock = MakeUnique<LockImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
+        if (try_lock && lock && !*lock) return {};
+        std::unique_ptr<Chain::Lock> result = std::move(lock); // Temporary to avoid CWG 1579
+        return result;
     }
     bool findBlock(const uint256& hash, CBlock* block, int64_t* time, int64_t* time_max) override
     {