Mark o3tl::heap_ptr as SAL_WARN_UNUSED

...to make it more straightforward to fix blunder as in
a88b2d841856026ea1a0539d670e07fafd296c6e "Resolves: fdo#81039 crash on adding
new macro library."

Conceivably, instances of heap_ptr could legitimately be created only for ctor/
dtor effect, but then again uses of heap_ptr should arguably be replaced with
std::unique_ptr anyway.

Change-Id: I9c765b8d265f3c26d459ec276e335eeebc8fee91

include/o3tl/heap_ptr.hxx

@@ -23,7 +23,7 @@
 
 #include <boost/assert.hpp>
 #include <boost/checked_delete.hpp>
-
+#include <sal/types.h>
 
 namespace o3tl
 {
@@ -72,7 +72,7 @@ namespace o3tl
     complain.
 */
 template <class T, class Deleter = boost::checked_deleter<T> >
-class heap_ptr
+class SAL_WARN_UNUSED heap_ptr
 {
   public:
     typedef T               element_type;       /// Provided for generic programming.

o3tl/qa/test-heap_ptr.cxx

@@ -138,6 +138,7 @@ class heap_ptr_test : public CppUnit::TestFixture
         {
             heap_ptr<Help>
                 t_raii( new Help(55) );
+            (void)t_raii;
             CPPUNIT_ASSERT_MESSAGE("raii1", Help::InstanceCount_() == nHelpCount + 1);
         }
         CPPUNIT_ASSERT_MESSAGE("raii2", Help::InstanceCount_() == nHelpCount);