Fix empty control statement warnings

[SigmundVik]

This change replaces code like this:

      if(safemode_or_autounlink)
         BOOST_INTRUSIVE_SAFE_HOOK_DEFAULT_ASSERT(node_algorithms::unique(to_insert));

with:

      BOOST_INTRUSIVE_SAFE_HOOK_DEFAULT_ASSERT(!safemode_or_autounlink || node_algorithms::unique(to_insert));

The reason for this change is that the first form generates a warning for Visual C++ 14.1 in builds without asserts:

warning C4390: ';': empty controlled statement found; is this the intent?

Before this change the code was using a mix of the two forms when invoking the BOOST_INTRUSIVE_SAFE_HOOK_DEFAULT_ASSERT() and BOOST_INTRUSIVE_INVARIANT_ASSERT() macros. Now it consistently uses the second form.

[SigmundVik]

Please pay special attention to the treap_node_extra_checker call operator change. If the node_traits::get_left() and node_traits::get_right() functions are allowed to have side effects, then we need to use curly braces to fix this case instead of moving the function calls inside the BOOST_INTRUSIVE_INVARIANT_ASSERT() macro invocation.

[igaztanaga]

Many thanks for the patch, it looks good, as get_rigth/get_left() is not suppossd to have side-effects