Fix (and test) that copy constructor decrements previous object counter

include/rcp.h

@@ -54,10 +54,18 @@ public:
 
     rcp<T> & operator=(const rcp & other)
     {
-        ptr = other.ptr;
-        if (ptr)
+        if (ptr != other.ptr)
         {
-            ptr->rcp_inc();
+            T * old_ptr = ptr;
+            ptr = other.ptr;
+            if (ptr)
+            {
+                ptr->rcp_inc();
+            }
+            if (old_ptr)
+            {
+                old_ptr->rcp_dec();
+            }
         }
         return *this;
     }