libCom: exitWait() from thread exit handler corrupts stack

The epicsThreadCallEntryPoint() function stores a pointer
to a local variable in epicsThread::pWaitReleaseFlag.
Calling epicsAtThreadExit::exitWait() from that thread's
epicsAtThreadExit() handler writes to this pointer
after epicsThreadCallEntryPoint() has returned.
Thus corrupting the stack.

Set pWaitReleaseFlag=NULL before return to prevent this.

fixes lp:1558206

src/libCom/osi/epicsThread.cpp

@@ -109,6 +109,7 @@ extern "C" void epicsThreadCallEntryPoint ( void * pPvt )
         // once the terminated flag is set and we release the lock
         // then the "this" pointer must not be touched again
     }
+    pThread->pWaitReleaseFlag = NULL;
 }
 
 bool epicsThread::beginWait () throw ()