Replaced use of while(isRunning()) { YieldCurrentThread(); } style loops with use of join() to avoid false positives being reported by valgrind when using the helgrind tool for thread debugging.

git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14460 16af8721-9629-0410-8352-f15c8da7e697

src/OpenThreads/pthreads/PThread.cpp

@@ -456,6 +456,10 @@ Thread::~Thread()
         // Kill the thread when it is destructed
         //
         cancel();
+
+        // wait till the thread is stopped before finishing.
+        join();
+
     }
 
     delete pd;