Address results of PVS-Studio static analysis

Couple are left un-addressed, see #958 for details.

include/internal/catch_matchers.cpp

@@ -0,0 +1,26 @@
+/*
+ *  Created by Phil Nash on 19/07/2017.
+ *
+ * Distributed under the Boost Software License, Version 1.0. (See accompanying
+ * file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_matchers.hpp"
+
+namespace Catch {
+namespace Matchers {
+    namespace Impl {
+
+        std::string MatcherUntypedBase::toString() const {
+            if( m_cachedToString.empty() )
+                m_cachedToString = describe();
+            return m_cachedToString;
+        }
+
+    } // namespace Impl
+} // namespace Matchers
+
+using namespace Matchers;
+using Matchers::Impl::MatcherBase;
+
+} // namespace Catch