replace remaining plain assert with kak_assert

src/buffer.cc

@@ -201,7 +201,7 @@ class UndoGroupOptimizer
                 ++pos.column;
             ++count;
         }
-        assert(pos == endpos);
+        kak_assert(pos == endpos);
         return count;
     }
 

src/utils.hh

@@ -24,7 +24,7 @@ public:
 
     static T& instance()
     {
-        assert (ms_instance);
+        kak_assert (ms_instance);
         return *ms_instance;
     }